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Image Formation by Spherical Mirror

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You might have observed that the image formed in the side view mirror of a car is smaller while the image formed in the rear-view mirror is larger than the actual size of the objects. The dentist uses a small mirror to examine the teeth of a patient. You might have also observed that for shaving purposes different mirrors are used. So, the different spherical mirrors are used for different purposes. We will discuss the spherical mirror in this article.

Spherical mirror is a type of mirror whose reflecting surfaces is the parts of a hollow sphere of glass.

Rules for Image Formation by Spherical Mirrors

Rule 1

  • For concave mirror: A ray of light parallel to the principal axis will pass through the focus after reflection.
  • For Convex mirror: A ray of light parallel to the principal axis will appear to come from focus.

Rule 1

Rule 2

  • For concave mirror: A ray passing through the principal focus(F) becomes parallel to the principal axis.
  • For convex mirror: A ray directed towards principal focus becomes parallel to the principal axis.

Rule 2

Rule 3

  •  For Concave mirror: A ray passing through the centre of curvature will take the same path after reflection.
  •  For convex mirror: A ray directed towards the centre of curvature will take the same path after reflection.

Rule 3

Rule 4

  • For any ray incident at any angle at the pole(P), the reflected ray follows the law of reflection for both concave as well as convex mirrors.

Rule 4

Image Formation by Concave Mirror

  • When an object is placed beyond C (centre of curvature): When an object AO is placed in front of a concave mirror, the object is placed beyond the centre of curvature of the concave mirror. The image formed after the reflection will be between the centre of curvature (C) and focus(F). The size of the image will be smaller than the object. The nature of the image will be real and inverted.
    • Image formed at – Between C & F
    • Image nature – Real & inverted
    • Image size – Smaller than the object

When an object is placed beyond C, then an image is formed between C & F.

  • When an object is placed at C: When an object AO is placed in front of a concave mirror, at the centre of curvature of the concave mirror. The image formed after the reflection will be at the centre of curvature (C). The size of the image will be equal to the object. The nature of the image will be real and inverted.                             
    • Image formed at –  At C
    • Image nature – Real & inverted
    • Image size – Same size as the object

When an object is placed at C, an image is also formed at C only.

  • When an object is placed between F & C: When an object AO is placed in front of a concave mirror, between the centre of curvature and the focus of the concave mirror. The image formed after the reflection will beyond the centre of curvature (C). The size of the image will be larger than the object. The nature of the image will be real and inverted.                             
    • Image formed at – Beyond C
    • Image nature – Real & inverted
    • Image size – Larger than the object

When an object is placed between F & C, the image is formed beyond C.

  • When an object is placed between F and P: When an object AO is placed in front of a concave mirror, between the focus (F) and the pole(P) of the concave mirror. The image formed after the reflection will be behind the mirror. The size of the image will be larger than the object. The nature of the image will be virtual and erect.
    • Image formed at – Behind the mirror
    • Image nature – Virtual and erect
    • Image size – Larger than the object

Image Formation by Convex Mirror

  • When an object is placed anywhere between ∞ and Pole: When an object AB is placed in front of a convex mirror at any point between Infinity (∞) and the pole (P). The image will always be formed behind the mirror. The nature of the image will be virtual and erect. The size of the image will be smaller than the object.
    • Image formed at – Behind the mirror
    • Image nature – Virtual & Erect
    • Image size – Smaller than the object

When an object is placed anywhere between ∞ and Pole, the image will form behind the mirror.

Sample Questions

Question 1: Riya stands in front of a spherical mirror. She finds that her image has a very fat body. Tell us what was the shape of the mirror?

Answer:

As the image has very fat body means she is standing in front of a concave mirror.

Question 2: Raj was standing in front of the mirror, he raised his right hand but in the mirror, he saw his left hand. The image is the same size as the size of his body. Tell us, what was the type of mirror in front of him. 

Answer:

As the size of the image is equal to size of his body and he saw his left hand right and vice versa. So, he was standing in front of a plane mirror.

Question 3: Two concave mirrors have the same focal length but the aperture of one is larger than that of the other. Which mirror forms the sharper image and why?

Answer:

As the concave mirror with smaller aperture is free from spherical aberration. So, concave  mirror with smaller aperture forms sharper image.

Question 4: Can we increase the range of a telescope by increasing the diameter of its objective?

Answer:

Yes, because the light gathering power of objective will increase and even faint objects will be visible. 

Question 5: A convex mirror is kept underwater. What would be the change in the focal length of the mirror?

Answer:

As the focal length of a spherical is independent of nature of  the medium. So, the focal length of the convex mirror doesn’t change.


Last Updated : 14 Mar, 2022
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