Focal Length Methods & Refractive Index — Practice Questions

Free NEET Physics multiple-choice questions on Focal Length Methods & Refractive Index. Attempt each question and reveal the answer with a full explanation.

In an experiment to verify the photoelectric effect, a plot of stopping potential V 0 against frequency is a straight line. The intercept of this line on the frequency axis represents: Threshold frequency Work function Planck's constant Stopping potential In a u-v experiment for a concave mirror, the object is placed at the center of curvature. The magnification produced is: -1 +1 -2 0 Two thin lenses of power +5 D and -2 D are placed in contact. A student wants to measure the focal length of the combination. The expected value of the focal length is: 33.3 cm 14.3 cm 20.0 cm 50.0 cm In the experiment to determine the refractive index of a glass prism, at the position of minimum deviation: The angle of incidence is equal to the angle of emergence The refracted ray is perpendicular to the base of the prism The angle of incidence is twice the angle of emergence The angle of deviation is equal to the angle of the prism To determine the refractive index of the material of a prism, the angle of minimum deviation is measured as 30 . If the angle of the prism is 60 , the refractive index is: 2 1.5 1.33 3 In a Zener diode regulated circuit, the Zener diode is always connected in: Reverse bias, in parallel with the load Forward bias, in parallel with the load Reverse bias, in series with the load Forward bias, in series with the load In the experiment to determine the refractive index of a glass slab using a traveling microscope, the formula used is = Real Depth Apparent Depth . If the readings for the bottom of the slab, the top of the slab, and a mark on top are d 1, d 2, and d 3 respectively, the real depth is: d 2 - d 1 d 3 - d 1 d 3 - d 2 d 1 - d 2 During an experiment to determine the focal length of a convex lens, a student plots a graph between object distance ( u ) and image distance ( v ). Which of the following graphs correctly represents this relationship for a real image? A rectangular hyperbola in the second and fourth quadrants of the u-v plane (considering sign convention) A straight line passing through the origin A parabola opening upwards A circle centered at the origin In a resonance tube experiment, the first resonance is heard at 16 cm and the second resonance is heard at 49 cm . The end correction of the tube is: 0.5 cm 1.0 cm 1.5 cm 2.0 cm In an experiment to measure the speed of sound using a resonance tube, if the frequency of the tuning fork is 512 Hz and the first resonance is at 15 cm (neglecting end correction), the speed of sound is approximately: 307 m/s 340 m/s 153 m/s 614 m/s In an experiment of photoelectric effect, the stopping potential ( V 0 ) is measured for different frequencies ( f ) of incident light. The slope of the V 0 versus f graph is equal to: h/e h e/h W 0/e During the determination of the focal length of a concave mirror, if the object is placed at 30 cm and the image is formed at 60 cm on the same side, the focal length is: -20 cm -90 cm -45 cm -15 cm In an experiment to determine the refractive index of a glass prism, a student measures the angle of the prism ( A ) and the angle of minimum deviation ( m ). If A = 60 and m = 30 , the refractive index of the glass is: 2 1.5 3 1.33 In a resonance tube experiment, the first resonance is obtained with a tuning fork of frequency 340 Hz . If the speed of sound is 340 m/s and the tube has a diameter of 4 cm , the first resonance length l 1 is: 23.8 cm 25.0 cm 26.2 cm 22.4 cm In the displacement method for a convex lens, if the distance between the two positions of the lens is 20 cm and the distance between the object and screen is 100 cm , the focal length of the lens is: 24 cm 21 cm 25 cm 16 cm A student is using a traveling microscope to measure the refractive index of a glass slab. The microscope is first focused on a mark on the table ( r 1 ). Then the slab of thickness t is placed over the mark and the microscope is refocused on the image of the mark ( r 2 ). Finally, it is focused on the top of the slab ( r 3 ). The refractive index is: (r 3 - r 1) / (r 3 - r 2) (r 3 - r 2) / (r 3 - r 1) (r 2 - r 1) / (r 3 - r 1) (r 3 - r 1) / (r 2 - r 1) In a resonance tube experiment, if the room temperature increases, the resonance length for the same tuning fork will: Increase Decrease Remain the same First increase then decrease In the measurement of the speed of sound using a resonance tube, why is it recommended to use a tuning fork of high frequency rather than low frequency? To reduce the length of the tube required and minimize errors in locating resonance To increase the loudness of the sound heard at resonance To avoid the effect of the end correction To ensure that the air column behaves like an ideal gas In a resonance tube experiment, the first resonance is heard at l 1 = 18 cm . If the water is replaced by an oil of higher density, the first resonance length l 1 will: Remain the same Increase Decrease Become zero A student measures the refractive index of a liquid using a traveling microscope. The microscope is focused on a mark at the bottom of a beaker (reading r 1 ), then focused on the same mark after pouring liquid (reading r 2 ), and finally focused on lycopodium powder sprinkled on the surface of the liquid (reading r 3 ). The refractive index is: (r 3 - r 1) / (r 3 - r 2) (r 2 - r 1) / (r 3 - r 1) (r 3 - r 2) / (r 3 - r 1) (r 3 - r 1) / (r 2 - r 1) In an experiment of photoelectric effect, the stopping potential for a certain metal is V 1 when illuminated with light of frequency 1 . If the frequency is doubled ( 2 1 ), the new stopping potential V 2 will be: More than 2V 1 Exactly 2V 1 Less than 2V 1 V 1 / 2 While using a traveling microscope to find the refractive index of a liquid, the microscope is focused on a mark at the bottom of an empty beaker (reading x ). After filling the liquid to height h , the microscope is refocused on the mark (reading y ). The apparent depth of the mark is: h - (y - x) y - x h + (y - x) (y - x) / h In the convex lens experiment to find focal length f using the u-v method, a plot of 1/u versus 1/v gives a straight line. The intercept on either axis is equal to: 1/f f 2/f f/2 During the determination of the refractive index of a liquid using a traveling microscope and a glass beaker, the formula used is = r 3 - r 1 r 3 - r 2 . Here r 2 represents the microscope reading when focused on: The image of the mark at the bottom as seen through the liquid The mark at the bottom of the empty beaker The surface of the liquid (lycopodium powder) The top edge of the beaker In a Zener diode as a voltage regulator, if the input voltage V in increases, what happens to the currents through the series resistor ( I s ), the Zener diode ( I z ), and the load resistor ( I L )? I s increases, I z increases, I L remains constant I s increases, I z remains constant, I L increases I s remains constant, I z increases, I L decreases All three currents increase While determining the focal length of a convex lens using the 'displacement method', if the distance between the two positions of the lens is d = 20 cm and the distance between the object and screen is D = 100 cm , the ratio of the sizes of the two images ( I 1/I 2 ) is: 2.25 1.50 4.00 1.25 Which of the following measurements require 'index correction'? Measurement of resistance of a wire using meter bridge Measurement of gravitational acceleration using simple pendulum Measurement of focal length of lenses using optical bench Measurement of speed of sound using resonance tube In a semiconductor diode experiment, as the forward bias voltage is increased, the width of the depletion layer: Decreases Increases Remains constant First increases then decreases To determine the angle of minimum deviation ( m ) for a glass prism, a student plots a graph between the angle of incidence ( i ) and the angle of deviation ( ). Which of the following is true at the minimum deviation position? The refracted ray inside the prism is parallel to the base of the prism The angle of incidence is half the angle of emergence The angle of emergence is zero The refracted ray is perpendicular to the base of the prism During the measurement of the focal length of a concave mirror using the 'u-v method', the object and the image coincide at a distance of 40 cm from the pole. The focal length of the mirror is: 20 cm 40 cm 80 cm 10 cm In the displacement method for finding the focal length f of a convex lens, the distance between the object and the screen is D . The two positions of the lens for which a sharp image is formed are separated by a distance d . The focal length is given by: f = D 2 - d 2 4D f = D 2 + d 2 4D f = D - d 4 f = Dd D+d For a thin prism with an angle of prism A , the refractive index is . The angle of minimum deviation m is approximately: ( - 1)A ( + 1)A A A / A convex lens forms a real image of an object. If the upper half of the lens is covered with black paper, then: The intensity of the image decreases Half of the image disappears The focal length of the lens changes The image shifts position