Question 1

In a plane mirror, if an object is 20 cm in front of the mirror, the image forms at?

Accepted Answer

20 cm behind the mirror. A plane mirror forms an image at the same distance behind the mirror as the object is in front. So an object 20 cm in front gives an image 20 cm behind. This image is virtual and cannot be obtained on a screen.

Question 2

Which statement expresses the second law of reflection?

Accepted Answer

The incident ray, reflected ray, and the normal lie in the same plane. The second law of reflection states that the incident ray, reflected ray, and the normal are coplanar. This means they all lie in the same plane at the point of incidence. The first law separately relates the two angles with the normal.

Question 3

A concave mirror has focal length 15 cm. Its radius of curvature is?

Accepted Answer

30 cm. For a spherical mirror, the radius of curvature is twice the focal length. So R = 2 × 15 cm = 30 cm. The center of curvature lies on the principal axis at this distance from the pole.

Question 4

An echo is heard clearly when the reflected sound reaches the ear after at least about?

Accepted Answer

0.1 s. The ear distinguishes two sounds separately only if the time gap is about 0.1 s or more. A reflected sound arriving after this delay is heard as an echo. This usually needs the reflector to be sufficiently far away in air.

Question 5

A convex mirror always forms an image that is?

Accepted Answer

Virtual, erect, and diminished behind the mirror. A convex mirror causes reflected rays to diverge, so they appear to come from behind the mirror. Therefore the image is virtual and erect. It is also diminished, giving a wider field of view.

Reflection — Set 1