NEET Physics: Physics - Optics questions with solutions

Q1. Select the odd one out w.r.t. the type of reflection of incident light (wood, steel, silver, mirror, graphite)

1. mirror
2. woodd
3. silver
4. steel

Answer: woodd

Wood has an irregular, rough surface, so it produces diffuse reflection rather than regular reflection. Mirror, silver, and steel are comparatively smooth and can give more regular reflection, making wood the odd one out.

Q2. A real image, equal in size to the object, is obtained when the object is placed at the centre of curvature in front of a:

1. Concave mirror
2. Plane mirror
3. Convex mirror
4. None of these

Answer: Concave mirror

A concave mirror can form a real image when the object is placed beyond its focal point, and at the centre of curvature the image forms at the same point with the same size. Plane and convex mirrors do not produce real images of this kind.

Q3. A planoconvex lens of focal length 20 cm has its plane side silvered This question has multiple correct options

1. The radius of curvature of curved surface of given plano-convex lens is equal to half of radius of curvature of a surface of equiconvex lens of focal length \( 20 \mathrm{cm} \)
2. An object placed at \( 15 \mathrm{cm} \) on the axis on the convex side of silvered plano-convex lens gives rise to an image at a distance of \( 30 \mathrm{cm} \) from it
3. An object placed at a distance of \( 20 \mathrm{cm} \) on the axis on the convex side of silvered plano-convex lens gives rise to an image at \( 40 \mathrm{cm} \) from it
4. silvered plano-convex lens acts as a concave mirror of focal length \( 10 \mathrm{cm} \)

Answer: silvered plano-convex lens acts as a concave mirror of focal length \( 10 \mathrm{cm} \)

For a plano-convex lens, the curved surface gives the full power; when the plane side is silvered, light refracts through the lens, reflects, and refracts again, doubling the lens effect. This makes the combination behave like a concave mirror with focal length half the lens focal length, i.e. 10 cm.