PHY 108 (PHY 192) Pat Questions and Answers
- When a light ray goes from air into glass, which happens to the angle with the normal?
A. It becomes smaller.
B. It becomes larger.
C. It stays the same.
D. It becomes zero.
Answer: A - Which measurement is essential when using a glass slab to find lateral displacement?
A. refractive index only
B. slab thickness and shift between emergent and incident rays
C. mass of the slab
D. focal length of the slab
Answer: B - For a concave mirror, an object placed at the focal point produces an image that is:
A. real and at 2f
B. virtual and magnified
C. formed at infinity (no real image on screen)
D. same size as the object
Answer: C - The lens formula (1/f = 1/u + 1/v) assumes:
A. small-angle (paraxial) approximation and thin lens
B. that the lens is thick only
C. only for mirrors, not lenses
D. no refraction occurs
Answer: A - In the displacement method for focal length, you keep object and screen fixed and move the lens. One advantage is:
A. you need only one measurement total
B. you can get two lens positions that give sharp image and use a direct formula for f
C. it eliminates the need for a lens
D. it requires laser light only
Answer: B - Minimum deviation through a prism is useful because:
A. it gives maximum dispersion
B. the refractive index can be calculated from apex angle and Dmin
C. it depends on the prism’s mass
D. it depends only on incident intensity
Answer: B - Which of the following is a sign of a well-balanced Wheatstone bridge?
A. maximum current through galvanometer
B. zero deflection of galvanometer
C. a flicker in galvanometer reading
D. rapid oscillation of the needle
Answer: B - A potentiometer measures EMF by:
A. drawing a steady current through the test cell
B. balancing against a known potential gradient so no current flows in the test cell at balance
C. heating the cell to a fixed temperature
D. comparing resistances only
Answer: B - Two identical resistors in parallel will have total resistance equal to:
A. twice one resistor
B. half one resistor
C. same as one resistor
D. zero
Answer: B - When using a galvanometer in a Wheatstone bridge, the sensitivity improves if the galvanometer has:
A. very high resistance and high coil sensitivity
B. zero resistance
C. no coil
D. very large mass
Answer: A - If you measure several (u, v) pairs for a convex lens and plot 1/u vs 1/v, the intercept gives:
A. 1/f
B. focal length f directly
C. object height
D. image brightness
Answer: A - In an experiment comparing EMFs of two cells with a potentiometer, the longer balancing length corresponds to:
A. the smaller EMF
B. the larger EMF
C. the cell with bigger diameter
D. the cell with lower temperature
Answer: B - Which practical step reduces random error when timing an optics alignment?
A. take several readings and average
B. use one reading only
C. ignore instrument readings
D. record measurements without units
Answer: A - Snell’s law at a plane surface is (n_1\sin\theta_1 = n_2\sin\theta_2). If (\theta_1) increases, (\theta_2) will:
A. decrease
B. increase (until total internal reflection if possible)
C. remain constant
D. become negative
Answer: B - In a concave mirror practical, the magnification m is +2. This indicates the image is:
A. virtual and upright, twice the object height
B. real and inverted, twice the object height
C. virtual and inverted
D. same size as object
Answer: A - When measuring the internal resistance of a dry cell by plotting terminal voltage V against current I, the internal resistance equals:
A. the y-intercept
B. the negative of the slope
C. the x-intercept
D. the slope itself (positive)
Answer: B - A student gets two different focal-length values for the same lens using different object distances. The most likely cause is:
A. measurement uncertainties and alignment errors
B. the lens physically changing f during the experiment
C. gravity altering optical power randomly
D. the light frequency changing spontaneously
Answer: A - Dispersion causes different colours to refract differently because:
A. refractive index depends on wavelength
B. speed of light in vacuum changes with colour
C. colours are produced inside the glass
D. prism adds energy to red light only
Answer: A - In a series circuit, if one resistor doubles in resistance, the total current (for fixed supply voltage) will:
A. increase
B. decrease
C. remain unchanged
D. become infinite
Answer: B - To reduce parallax when reading a scale in a lab, you should:
A. position your eye perpendicular to the scale mark
B. read from a steep angle for clarity
C. move the scale while reading
D. close one eye and tilt the scale randomly
Answer: A - If the galvanometer in a Wheatstone bridge is very sensitive, balancing the bridge requires:
A. larger changes in ratio values
B. very small adjustments to reach zero deflection
C. removing the galvanometer
D. reversing the whole circuit
Answer: B - For a thin lens, when object distance u equals image distance v, the object position is:
A. at 2f (twice focal length)
B. at f (focal point)
C. at infinity
D. on the lens surface
Answer: A - Which arrangement is best to measure resistor R with least systematic error using ammeter and voltmeter?
A. voltmeter with high resistance across R, ammeter with low resistance in series with R
B. voltmeter in series and ammeter in parallel
C. remove both instruments and estimate R visually
D. connect voltmeter across power supply only
Answer: A - In practice, a tungsten filament’s resistance increases when current flows because:
A. filament temperature rises, increasing resistivity
B. filament gets polarized and reduces resistance
C. resistance is independent of temperature
D. the filament length decreases with current
Answer: A - When measuring the refractive index of a prism using minimum deviation, you must accurately measure:
A. apex angle and minimum deviation angle
B. prism mass and thickness
C. ambient humidity and prism color
D. prism temperature only
Answer: A - A student uses white light for lens focal experiments and gets fuzzy images. The likely reason is:
A. chromatic aberration — different colours focus at slightly different points
B. white light has no photons
C. lenses do not work with white light
D. the lens dissolved in white light
Answer: A - The balanced condition of Wheatstone bridge for unknown Rx is (R_x = (R_2/R_1)\times R_3). If R1 and R2 are interchanged, correct action is to:
A. recalculate Rx using the new ratio
B. ignore interchange — nothing changes
C. remove Rx from circuit entirely
D. short the bridge wires
Answer: A - In a potentiometer, the potential gradient along the wire is defined as:
A. voltage drop per unit length of the wire
B. current per unit length
C. resistance per unit length only
D. mass per unit length
Answer: A - If you connect two identical cells in parallel, the total EMF and internal resistance approximately become:
A. EMF = E, r = r/2
B. EMF = 2E, r = 2r
C. EMF = E/2, r = r/2
D. EMF = 0, r = infinite
Answer: A - When locating a real image from a convex lens onto a screen, the sharpest image indicates:
A. the true image plane where v is correctly measured
B. a random spot independent of lens position
C. the lens has no focal length
D. the object is at the lens center
Answer: A - In an optics lab, why use a monochromatic source like sodium lamp for lens/prism measurements?
A. to avoid chromatic spread and get sharp minima or images
B. because sodium lamps are cheaper than white lamps always
C. because monochromatic light increases lens focal length
D. to heat the lens for better performance
Answer: A - For two resistors R1 and R2 in series, the voltage across R2 equals:
A. (V_{total}\times(R_2/(R_1+R_2)))
B. (V_{total}\times(R_1+R_2))
C. (V_{total}) only if R1 = 0
D. half of R1 always
Answer: A - Which error reduces accuracy when using a potentiometer to find EMF?
A. poor contact at jockey or non-uniform wire
B. reading values in correct units
C. using a calibrated standard cell
D. balancing to zero deflection properly
Answer: A - In a graph of V (y) vs I (x) for a cell with internal resistance, the y-intercept equals:
A. the EMF of the cell
B. internal resistance r
C. negative of r
D. zero always
Answer: A - Which observation indicates total internal reflection at an interface?
A. no refracted ray emerges and light is completely reflected back into the denser medium
B. light refracts out into the rarer medium strongly
C. light is absorbed at the boundary
D. light passes through unaffected
Answer: A - To find focal length by the autocollimation method, you need:
A. a plane mirror and an object — lens adjusted until image coincides with object (sharp), then distance = f
B. only a prism and no lens
C. a voltmeter and ammeter only
D. two identical lenses in contact only
Answer: A - In the tungsten filament experiment, plotting V/I vs I shows a rising trend because:
A. resistance increases with current (temperature) so V/I increases
B. resistance decreases with temperature so V/I falls
C. V/I is constant under all conditions
D. current is independent of voltage
Answer: A - When calibrating a potentiometer, you must ensure the driver cell is:
A. stable and has higher EMF than the test cells
B. equal to zero volts
C. variable randomly during measurement
D. disconnected after every reading
Answer: A - If the object for a concave mirror is between the focus and the mirror, the image formed is:
A. virtual, upright and magnified
B. real and inverted on the screen
C. same size and inverted
D. not formed at all
Answer: A - A practical way to improve Wheatstone bridge accuracy is to:
A. use decade resistors or precise standard resistors close in value to the unknown
B. use wildly different resistor orders of magnitude
C. remove the galvanometer entirely
D. use only variable capacitors instead
Answer: A - What happens to the lateral shift through a glass slab if the slab thickness doubles (incident angle fixed)?
A. lateral shift roughly doubles (proportional to thickness)
B. lateral shift halves
C. lateral shift remains zero
D. lateral shift becomes negative
Answer: A - In an electricity lab, why place the voltmeter in parallel with the component under test?
A. it measures the potential difference across that component directly without interrupting current path
B. it measures current only in that configuration
C. it reduces the circuit resistance to zero
D. it converts voltage to frequency
Answer: A - While finding prism minimum deviation, turning the prism slowly and monitoring the emergent ray’s movement helps because:
A. the minimum deviation is where emergent ray changes direction sign (slowest movement) — easy to identify
B. it increases prism temperature to improve results
C. it causes chromatic inversion
D. it creates resonance in the prism
Answer: A - In a series circuit, the same current flows through each resistor because:
A. there is only one path for charge flow
B. each resistor has identical voltage across it always
C. current splits equally by law
D. resistors force current to change at each junction
Answer: A - Which of these is true for the image formed by a convex lens when the object is at 3f?
A. the image is real, inverted and between f and 2f (smaller than object)
B. image is virtual and erect
C. image is at infinity always
D. image equals object size and at 3f on other side
Answer: A - When using a potentiometer wire that is dusty and dirty, you may get:
A. unstable contact and incorrect balance lengths
B. more accurate readings
C. increased wire conductivity uniformly improving gradient
D. automatic calibration of the potentiometer
Answer: A - A student wants to test Ohm’s law for a resistor and sees the V–I plot curves upward (non-linear). The correct interpretation is:
A. resistor is non-ohmic (resistance changes with V or temperature)
B. Ohm’s law is false universally
C. the resistor is superconducting
D. the voltmeter is measuring frequency instead
Answer: A - If the galvanometer coil is damped heavily during Wheatstone balancing, the needle will:
A. return to rest quickly with little oscillation (good for fast balancing)
B. oscillate forever
C. never move
D. melt the coil
Answer: A - During lens experiments, why note the temperature or room conditions?
A. because lens material refractive index and instrument readings can shift slightly with temperature (affects precision)
B. because lenses change focal length drastically day by day
C. because room color alters refractive index significantly
D. because temperature determines lens polarity
Answer: A - Best immediate safety practice before rearranging connections in a circuit is to:
A. switch off the power supply and discharge capacitors if present
B. touch live wires to ensure current is present
C. keep power on for convenience
D. use metal tools across live nodes to test continuity
Answer: A - In the displacement method to find a lens’s focal length, which pair of measurements is needed?
A. lens thickness and room temperature
B. two lens positions that form a sharp image on a fixed screen and the separation between them
C. the colour of the light and the lens material
D. object mass and image brightness
Answer: B - Which best describes chromatic aberration in a lens?
A. it makes the lens heavier
B. it is corrected by lowering temperature
C. different wavelengths focus at slightly different distances from the lens
D. it causes total internal reflection
Answer: C - Why use a sodium lamp for a prism experiment?
A. sodium lamps are red only
B. white light is too bright
C. the prism works only with sodium
D. monochromatic light gives sharp minimum deviation and avoids colour spread
Answer: D - If object distance (u=30) cm and image distance (v=15) cm, what is the focal length (f) using (1/f=1/u+1/v)?
A. 5 cm
B. 20 cm
C. 45 cm
D. 10 cm
Answer: A - In a concave mirror experiment, an image that is real, inverted and smaller than the object suggests the object is:
A. between mirror surface and focal point
B. beyond centre of curvature (beyond 2f)
C. at the focal point
D. at the centre of curvature
Answer: B - Which procedure reduces systematic error when using a potentiometer?
A. use the shortest possible wire length only
B. balance with current flowing through the test cell
C. ensure good, clean contact with the jockey and use a stable driver cell
D. use a fluctuating driver voltage
Answer: C - For a balanced Wheatstone bridge with (R_1/R_2 = R_3/R_x), the correct expression for (R_x) is:
A. (R_1 + R_2 – R_3)
B. ((R_3 R_2)/R_1)
C. ((R_1 R_3)/R_2)
D. (R_3 – R_1)
Answer: B - When plotting terminal voltage (V) vs current (I) for a loaded cell, the y-intercept equals:
A. internal resistance
B. short-circuit current
C. zero
D. EMF of the cell
Answer: D - For a lens with object distance (u=40) cm and image distance (v=80) cm, the image is:
A. inverted and twice the size of the object
B. upright and half the size
C. inverted and half the size
D. virtual and same size
Answer: A - Adding another resistor in parallel to an existing parallel network will:
A. keep resistance same if all resistors identical
B. increase total resistance
C. make the circuit open
D. decrease total resistance
Answer: D - To avoid parallax when reading a metre rule during optics alignment you should:
A. read from an angle for speed
B. move the rule while reading
C. close both eyes and estimate
D. align your eye perpendicular to the scale mark while keeping the rule steady
Answer: D - The prism refractive index (n) is given by which formula (with apex angle (A) and minimum deviation (D_{min}))?
A. (n = \sin\big(\tfrac{A}{2}\big)/\sin\big(\tfrac{A+D_{min}}{2}\big))
B. (n = \tan(A+D_{min}))
C. (n = \sin\big(\tfrac{A+D_{min}}{2}\big)/\sin\big(\tfrac{A}{2}\big))
D. (n = A/D_{min})
Answer: C - If a fixed object and screen are separated by distance (L) and the lens has two sharp positions separated by (d), the focal length is:
A. (f = \dfrac{d^2}{L})
B. (f = \dfrac{L-d}{2})
C. (f = L + d)
D. (f = \dfrac{L^2}{4d})
Answer: D - Total internal reflection is used in the lab primarily to:
A. change mass of the medium
B. produce chromatic dispersion only
C. create EMF in wires
D. measure the refractive index of liquids by finding the critical angle
Answer: D - To minimize measurement error when using an ammeter and voltmeter, choose:
A. voltmeter in series and ammeter in parallel
B. remove both instruments and estimate R visually
C. use an ammeter with low resistance and a voltmeter with high resistance
D. connect voltmeter across power supply only
Answer: C - Why does a tungsten filament’s resistance increase when current flows?
A. its length doubles only
B. electrons disappear
C. its resistivity increases with temperature
D. applied voltage ceases to exist
Answer: C - If someone reports a focal length with no uncertainty, the examiner will likely:
A. accept it as perfect data
B. award full marks always
C. ask for uncertainty or mark down for incomplete reporting
D. ignore units only
Answer: C - Which method can directly give the refractive index of a liquid like groundnut oil using lens techniques?
A. guessing from smell
B. using a thermometer only
C. weighing the oil only
D. measuring focal length change when a lens is immersed and applying proper corrections
Answer: D - Why are multiple trials recommended in a well-run practical?
A. to create more pages for the report
B. to estimate random error and improve confidence in the mean value
C. to increase the value of the instrument
D. to confuse students
Answer: B - Jitter at the potentiometer null point most likely indicates:
A. perfect experimental setup
B. unstable driver supply or poor contact at the jockey
C. the null point always moves with time by law
D. humidity alone makes potentiometers impossible
Answer: B - The (approximate) lens-maker’s formula for a thin lens in air is:
A. (f = R_1 + R_2)
B. (n = f(R_1-R_2))
C. (\dfrac{1}{f} = (n-1)\left(\dfrac{1}{R_1} – \dfrac{1}{R_2}\right))
D. (R_1R_2 = n)
Answer: C - In a Wheatstone bridge, using standard resistors close in value to the unknown helps because:
A. it warms up the circuit
B. it neutralizes galvanometer sensitivity
C. it increases balancing accuracy and lowers percentage error
D. it always reduces the unknown value
Answer: C - A converging lens produces a virtual erect image when the object is placed:
A. at infinity
B. beyond 2f
C. exactly at 2f
D. between lens and focal point (inside f)
Answer: D - If you smell burning plastic from a circuit, the best immediate action is to:
A. continue experimenting for data completeness
B. switch off the power supply and inform the instructor
C. use wet hands to touch components
D. plug in more devices to test overload
Answer: B - When plotting (1/v) vs (1/u) for a lens, the intercept helps determine:
A. focal length from intercept (1/f) and check linear behaviour
B. mass of the lens
C. colour of the image
D. EMF of a cell
Answer: A - Compared to a single cell case (neglecting internal resistance), connecting two identical cells in series will make the circuit current:
A. unchanged
B. halved always
C. zero
D. doubled approximately
Answer: D - A clear null in a potentiometer balance indicates:
A. the driver cell has zero voltage always
B. the circuit is shorted deliberately
C. the balance point where no current flows through the test cell (ideal null)
D. the potentiometer is broken
Answer: C - For prism refractive-index measurements, the most critical reading is:
A. the prism’s weight
B. the colour of the table in lab
C. the time of day only
D. the angle of minimum deviation measured precisely
Answer: D - In a series circuit, the voltage drop across each resistor depends on:
A. resistor colour only
B. nothing — it is always equal for every resistor
C. the flavour of wires used
D. its resistance relative to total resistance and the supply voltage
Answer: D - Why is the null method (potentiometer) preferred for precise EMF measurement?
A. it uses a magnet to measure voltage only
B. it is faster to guess values
C. it always gives zero EMF
D. it avoids drawing current from the test cell at balance, eliminating loading errors
Answer: D - Doubling the thickness of a glass slab (same incident angle) will:
A. make the lateral shift negative
B. halve the lateral shift
C. keep lateral shift zero
D. roughly double the lateral shift (proportional to thickness)
Answer: D - If the galvanometer needle deflects slightly even at balance, you should:
A. accept and record the deflection as balance
B. check and improve contacts, zero-set the galvanometer, or use a more sensitive null method
C. heat the galvanometer to reduce sensitivity
D. remove the bridge wires entirely
Answer: B - For a concave mirror, if object distance (u = f), the image formed is:
A. at 2f and real
B. virtual and small
C. formed at infinity (rays emerge parallel)
D. disappears always
Answer: C - A non-ohmic device is identified by:
A. infinite resistance only
B. perfect straight-line V–I graph at all conditions
C. zero current always
D. a nonlinear V–I graph (resistance changes with V or temperature)
Answer: D - Best practice when reporting lab results is to:
A. include numerical value ± uncertainty and units, describe method and main error sources
B. give only the final number with no context
C. use inconsistent units in same report for variety
D. produce results without stating instruments used
Answer: A - To check the lens axis is straight on an optics bench, you should:
A. guess by eye without tools
B. use a voltmeter across the lens
C. rotate the whole bench randomly
D. use an optical bench with collimated source and ensure object, lens, and screen centres line up on the same axis
Answer: D - If minimum deviation for red light is smaller than for violet in a prism, it means:
A. violet is not affected by glass
B. red light is heavier than violet light
C. colours don’t matter in dispersion
D. red light has lower refractive index in the prism material than violet (normal dispersion)
Answer: D - To estimate the internal resistance of a cell without shorting it, you should:
A. short the cell directly and measure heating
B. insert the cell into a microwave
C. measure terminal voltage at several known loads, plot V vs I and extract r from slope
D. measure the cell’s mass exactly
Answer: C - In a V–I test, a straight-line plot slope of 5 Ω represents:
A. the power at any voltage
B. the current squared
C. the conductance only
D. the resistance of the resistor (5 Ω)
Answer: D - To reduce random error when balancing a Wheatstone bridge you should:
A. change resistors mid-balance randomly
B. use only one coarse knob always
C. take several balance readings and average the corresponding ratio values
D. fail to make any adjustment and record the first reading
Answer: C - When a convex lens is placed in a liquid with refractive index close to the lens material, the focal length will:
A. become undefined by law
B. invert sign randomly
C. decrease to zero always
D. increase (lens power drops)
Answer: D - Why use a plane mirror in the autocollimation method?
A. to heat the lens for calibration
B. to measure current precisely
C. to change the focal length magically
D. to reflect emergent rays back through the lens so a sharp coincident image occurs at the object when lens is at focal distance
Answer: D - If the driver cell voltage falls during a potentiometer run, the measured null lengths will:
A. increase randomly only
B. be unaffected always
C. become imaginary numbers
D. shorten proportionally, introducing systematic error unless corrected
Answer: D - Wide slits producing fuzzy diffraction patterns are fixed by:
A. using a different colour of bench
B. removing the slit entirely
C. making the slit wider for more fuzziness
D. narrowing the slit or using a more collimated source to produce clearer fringes
Answer: D - Galvanometer internal damping helps by:
A. changing resistance of bridge automatically
B. producing larger oscillations for style
C. preventing any movement at all always
D. reducing oscillation so the pointer settles faster for easier null detection
Answer: D - A large relative error in (v) when (v \gg u) during focal-length calculations will:
A. reduce the lens focal length physically
B. cancel out exactly
C. have no effect at all
D. significantly affect calculated (f) (propagated error larger)
Answer: D - “Zero deflection” in a potentiometer or bridge practically means:
A. the circuit is shorted completely
B. the instrument is dead and unusable
C. the unknown becomes zero always
D. no current flows through the detector — ideal balance condition
Answer: D - Why is a sodium lamp preferred for prism minimum deviation?
A. it is the cheapest light source in lab supplier list
B. it emits all colours equally so averaging is easier
C. it lowers the prism temperature for accurate mass reading
D. its nearly single wavelength avoids multiple minima and gives a sharp, repeatable minimum deviation reading
Answer: D - Before connecting the circuit to measure current through a filament, which is important?
A. use wet hands to make better contact
B. over-tighten wires to break them intentionally
C. always short the power supply momentarily to test fuses
D. ensure connections are secure, instruments rated for expected current, and power initially set low
Answer: D - Which lab habit improves every experiment?
A. skipping notes and relying on memory
B. coloring the apparatus for flair only
C. doing experiments at midnight only
D. careful notes (values, uncertainties, conditions), neat diagrams, and a short conclusion stating major error sources and suggestions for improvement
Answer: D
