Question

A student obtains a sharp image of the distant window $\left( W \right)$ of the school laboratory on the screen $\left( S \right)$ using the given concave mirror $\left( M \right)$ to determine its focal length. Which of the following distances should he measure to get the focal length of the mirror?
 

A. $MW$
B. $MS$
C. $SW$
D. $MW - MS$

Answer 1

Hint:It is considered a concave mirror if the inner side of the spherical mirror is reflective. A magnified and virtual image is acquired when the concave mirror is located very close to the object. However, if we increase the distance between the object and the mirror, the size of the image will decrease and the actual image will be produced. The image created by the concave mirror can, thus, be small or large and can be real or virtual as well.

Complete answer:
The rules for image formation by a concave mirror are: Any incident ray that passes parallel to the main axis on the way to the mirror will pass through the focal point upon reflection. On the path to the mirror, any incident ray passing through the focal point will travel parallel to the main axis upon reflection.

In a concave mirror when the object is at infinity the position of the image is at the focus.In a concave mirror when the object is beyond the centre of curvature the position of image is between the focus and centre of curvature.In a concave mirror when the object is at the centre of curvature the position of image is also at the centre of curvature.

In a concave mirror when the object is between the centre of curvature and focus the position of the image is beyond the centre of curvature. In a concave mirror when the object is at focus the position of the image is at infinity.Hence, we can see that the concave mirror forms the image of a distant object in its focus. Thus, from the diagram, MW is the distance of the object, while MS is the distance of the reflection equal to the focal length of the mirror. In order to assess the focal length of the mirror, the student needs to estimate MS.

Therefore, option B is correct.

Note:Here we have to observe where the image is placed in the diagram and also what type of mirror is given.Moreover,a mirror is a reflective surface that bounces off light-producing either a real image or a virtual image. When an object is placed in front of a mirror, the image of the same object is seen in the mirror. The object is the source of the incident rays and the image is formed by the reflected rays. Based on the interaction of light, the images are classified as either a real image or a virtual image.