Greeshma has a mirror and a lens with her. She found that both of them had a focal length of (-) 13 cm. Identify the type of mirror and lens Greeshma has.

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Detailed Solution

We have been given a focal length = (-) 13cm.
1) A concave lens causes the parallel light beams that strike it to diverge. A concave mirror has an inward curving reflected surface.
2) The direction of incident light is regarded as positive in optics. Focus is positioned at the front of the reflecting surface.
3) Figure 1 represents that the incident ray travels in a positive direction before reflection, but it changes direction after passing through the focus to travel in a negative direction. Consequently, a concave mirror has a negative focal length.
4) The focus is positioned on the same side as the incident light in a diverging lens. As shown in figure 2, the light rays refracted from the point on the other side of the lens diverged.

Conventionally, the focal length of a concave mirror and a concave lens is assumed to be negative. As a result, the spherical mirror and a thin spherical lens are both concave in nature.