Light - Reflection and Refraction: questions with answers

The angle of incidence is 0 degrees since the ray falls along the normal. The angle of reflection always equals the angle of incidence, so it is also 0 degrees.

A plane mirror always forms a virtual, erect, same-sized image of an object placed in front of it. The image is virtual because it cannot be caught on a screen; it only appears to lie behind the mirror.

When an object lies between the pole and the focus of a concave mirror, the reflected rays diverge and never actually meet in front of the mirror. The virtual image is located where the backward extensions of these reflected rays meet, behind the mirror.

The ray strikes the mirror along the normal, which passes through the centre of curvature. Its angle of incidence is zero, so it reflects back along its incoming path.

The centre of the sphere of which the mirror is a part is called the centre of curvature. A learner who mistakes it for the pole thinks it lies on the mirror surface, but it lies away from the surface, on the principal axis.

A mirror is named by the shape of its reflecting surface, not its back. A surface that bulges outward toward the object is convex.

For an object placed beyond C, the concave mirror forms a real, inverted and diminished image lying between F and C, not at F itself.

Between P and F, the image is virtual, erect and enlarged behind the mirror. Beyond C the image is real, inverted and diminished; between C and F it is real, inverted and enlarged. Neither case is virtual, so the claim that a concave mirror always gives a real image is false.

A convex mirror image of a real object is always virtual, erect and diminished behind the mirror. A highly diminished image appears near F.

A convex mirror always gives an erect, diminished image, so it shows a much wider area behind the vehicle.

Use $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$ with $f = -10$ cm and $u = -20$ cm. So $\frac{1}{v} = \frac{1}{-10} - \frac{1}{-20} = -\frac{1}{20}$, giving $v = -20$ cm.

When light passes from one medium into another and changes direction at the boundary, this is refraction. Reflection instead sends light back into the first medium; it does not describe a ray that crosses the surface and continues into the glass.

Speed in a medium equals c divided by n. Here v = 3.0×10^8 m/s ÷ 2.5 = 1.2×10^8 m/s. A higher refractive index means a slower speed, not a faster one.

A lens that is thinner at the centre than at the edges is concave and spreads out parallel rays as if from a point behind it. A convex lens, thicker at the centre, would converge the rays instead.

Use the signed lens formula 1/f = 1/v - 1/u. Here 1/u = 1/v - 1/f = -1/10 - (-1/15) = -1/30, so u = -30 cm.