Question

(a) A mobile phone lies along the principal axis of a concave mirror. Show, with the help of a suitable diagram, the formation of its image. Explain why magnification is not uniform.
(b) Suppose the lower half of the concave mirror reflecting surface is covered with an opaque material. What effect will this have on the image of the object? Explain.

Answer 1

Hint:To solve this question, we need to know the types of images formed by the concave mirror. As the location of the object changes, the size and nature of the image also changes. We will use this concept for answering the first question. In the second question, an opaque material is used to cover the lower half of the concave mirror. It will certainly produce the image but with some variations.

Complete answer:
(a) First, we will give the answer of the first question by explaining it using the following diagram.

As shown in the diagram, a mobile phone lies along the principal axis of a concave mirror. It is clearly observed that the portion of the mobile phone which is at C will form an image of the same size only at C. We can see that the height of one end of the mobiles remains the same.

Now, the portion of the mobile phone which lies between C and F will form an enlarged image beyond C as shown in the diagram.We can notice that the magnification of each part of the mobile phone cannot be uniform because of the different locations. This is the reason why the image formed is not uniform.

(b) Now, we will answer the second question.We know that for all points of the mirror, the laws of reflection are true. Therefore, the height of the whole image will be produced. However, the intensity of the image will be decreased as the area of the reflecting surface has been decreased.

Note:There are two important things to note in this question. First, we have seen that only the part of the mobile which lies on the centre of the curvature of the concave mirror has an exact size of image. Second, by covering the lower half of the mirror, the intensity of the image is decreased. This means that the image will be less bright in this case.