Optics Questions and Answers

Answer: B

The suitable explanation is that light travels in a straight line.

Light from each point of the object passes in a straight line through the tiny pinhole and falls on a single point of the screen. Rays from the top of the object reach the bottom of the screen and rays from the bottom reach the top, so a real, inverted image forms on the screen.

Diffraction at the small hole would only blur the image, not create it. That light is a form of energy is true but explains nothing about image formation, and refraction (bending when passing into another medium) plays no part in a pinhole camera.

Answer: D

(A) is true and only (R2) is the correct explanation for (A).

Convex mirrors are used as rear-view mirrors because they give a wider field of view, letting the driver see more of the traffic behind.

(R1) is false: a convex mirror always forms an erect but diminished image, never an enlarged one. So it cannot explain (A), and options that accept (R1) are wrong.

Answer: D

The conclusion is that the refractive indices of A and C are less than that of B.

Refractive index is inversely related to the speed of light in a medium. Light slows on entering B from A, so B has a higher refractive index than A. It speeds up again on entering C, so C has a lower refractive index than B.

Nothing tells us how A and C compare with each other, so 'equal to C' cannot be concluded, and A is not more than B. The information does allow comparison with B, so option 1 is wrong.

Answer: C

The answer is A-No change; B-Increases; C-Decreases; D-No change.

When light passes from one medium into another, its frequency is fixed by the source and does not change. Speed and wavelength change together, since

\[ \begin{aligned} v &= f\lambda \\ \lambda &\propto v \ \text{when } f \text{ is fixed} \end{aligned} \]

In X the speed increases, so frequency shows no change (A) and wavelength increases (B). In Y the wavelength decreases, so the speed decreases (C) and the frequency again shows no change (D). Options A and D let frequency change, and option B has the wavelength falling while speed rises.

Answer: C

Group A can make a kaleidoscope and Group B a periscope.

A kaleidoscope is made by fixing three rectangular glass (mirror) strips into a prism inside a cardboard tube, closing one end with a circular glass plate holding pieces of broken bangles, and the other end with a cardboard disc with a peephole. Multiple reflections in the strips produce the symmetrical patterns. These are Group A's materials.

A periscope uses two plane mirrors fixed at 45° at the two ends of a tube or box, which is exactly Group B's set. Newton's disc needs a coloured disc and a spinning device, and a pinhole camera needs no mirrors, so options A, B and D do not fit.

Answer: A

Such an object is called opaque.

An opaque object, such as wood, a brick or a metal sheet, does not let any light pass through it, so we cannot see through it and it casts a dark shadow.

A translucent object, such as butter paper or frosted glass, lets some light through but scatters it, so objects behind it look blurred. A transparent object, such as clear glass, lets almost all light through. Convex describes the shape of a lens or mirror, not how much light passes.

Answer: C

The distance between Manjula and her image is 8 m.

A plane mirror forms the image as far behind the mirror as the object is in front of it. After moving 1 m closer, Manjula is 5 − 1 = 4 m from the mirror, so her image is 4 m behind it.

\[ \text{distance} = 4 + 4 = 8\ \text{m} \]

4 m is only her distance from the mirror, 10 m is the original distance before she moved, and 1 m is how far she moved.

Answer: B

The correct statements are A, C, D.

The image in a plane mirror is erect (upright), laterally inverted (left appears as right), the same size as the object, and as far behind the mirror as the object is in front of it.

Statement B is wrong: the image is virtual, not real. Light rays only appear to come from behind the mirror, so the image cannot be caught on a screen. Every option containing B is therefore wrong.

Answer: D

The order from top to bottom is concave, plane, convex.

A concave mirror gives an enlarged, upright image when the object is close to it, within its focal length, so the head looks bigger. A plane mirror gives an image of the same size, so the middle of the body looks normal. A convex mirror always gives a diminished, upright image, so the legs look smaller.

Options with a convex mirror at the top would make the head look smaller, not bigger, and an option with a convex mirror in the middle would shrink the middle of the body instead of showing it at its true size.

Answer: C

The correct option is X - Decreases; Y - Remains unchanged; Z - Increases; W - Remains unchanged.

When light passes from one medium to another, its frequency never changes; it is set by the source. So Y and W are both 'remains unchanged'. Since speed = frequency × wavelength, the speed and the wavelength change together, in the same direction.

Into medium A the wavelength decreases, so the speed decreases too (X). Into medium B the speed increases, so the wavelength increases too (Z). Every option in which the frequency changes is therefore wrong, which leaves only X - Decreases; Y - Remains unchanged; Z - Increases; W - Remains unchanged.