Convex Mirror and Lens

Mirror mirror on the wall, who is the fairest of them all. Science chapter on Reflection of Light proves that mirrors are much more purposeful than just reflecting our beauty. In fact, mirrors can perform a variety of tasks ranging from everyday use to scientific experiments. There are two types of curved mirrors- Convex and Concave with different properties and use. Similarly, we have 2 kinds of lenses, convex and concave. Understanding the basic working of mirrors and lenses can be confusing for most of us. But as these topics are an important part of class 8th – class 12th Science, we cannot afford to miss them. Here is a comprehensive blog that explains the uses and properties of convex mirrors and lenses well!

What is Convex Mirror?

A curved mirror in which the reflective surface bulges out towards the source of the light is called a convex mirror. The bulged-out surface reflects the light outwards and is not used to focus the light. It has an imaginary Focal Point (F) and the Centre of Curvature (2F) which cannot be reached hence, the mirror forms a virtual image. The images formed through a convex mirror cannot be projected on a screen as it is contained inside the mirror. The image is smaller in size as compared to the object but gets larger when the object gets closer to the mirror. Now that you are clear with what a convex mirror is, let us understand its working of it:

In the above-given image, the centre of the curvature (C) is the centre of the original sphere from which the mirror is taken and the line that passes through the centre of the sphere and from the mirror’s surface is called the Principal axis. (F) is the Focal point of the mirror which is located on the principal axis and is exactly at the middle point between the mirror’s surface and the centre of curvature. The Centre of curvature (C) and the focal point (F) are always located on the unpolished side of the mirror which is opposite to the image. Due to this, the convex mirror has a negative focal length value.   

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Images formed by Convex Mirror

A convex mirror can only form 2 types of images when the object is placed at different positions. The two possibilities are- when the object is at infinity and the object is between the pole and infinity. Let us have a look at the characteristics of the images formed at these two positions. 

• Object at the infinity

If an object is at the infinity then, at the principal focus a point sized image is formed which is highly diminished, virtual and erect.

• Object between infinity and the pole 

When the object is placed between infinity and the pole then, a diminished, virtual and erect image is formed between the pole and focus of the convex mirror.  

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What is Convex Lens?

A convex lens is thicker at the centre and thinner at the edges. An optical lens is made up of two spherical surfaces which are curved outwards called the biconvex lens or convex lens. Such lenses can converge the ray of light falling on them and focus them at a point on the other side called the focus. The distance between the lens and the focus is called the Focal length of the lens. A convex lens varies a lot from convex mirror. Let us have a look at the various types of images formed by a convex lens.

Images formed by Convex Lens 

Whenever we bring an object closer to the convex lens, the image gets enlarged. Following is a detailed analysis of the types of images formed-

• Infinity 

• Beyond 2f

• Between 2f and f

• At f 

• At 2f

• Between f and 0

Tabulated below are the features of the images formed using a convex lens-

Object Position	Image Position	Nature	Size
Infinity	At 2f	Real and Inverted	Diminished
Beyond 2f	Between 2f and f	Real and Inverted	Diminished
Between 2f and f	Beyond 2f	Real and Inverted	Diminished
At f	At infinity	Real and Inverted	Enlarged
At 2f	At 2f	Real and Inverted	Same Size
Between f and 0	On the same side as that an object	Virtual and Erect	Enlarged

Differences Between a Convex Mirror and Convex Lenses

Here are the differences between a convex mirror and convex lenses:

• The convex lens is transparent. But the convex mirror is opaque.
• The convex lens is dependent on the refraction of light but the convex mirror is responsible for the reflection of light.

Questions Based on Convex Mirror and Lens

1. A Pen is placed 4 cm high and at a distance of 10 cm from a convex lens of focal length 20cm. Can you identify the position, type and size of the image formed?
2. A concave lens has a focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Calculate the size of the image formed.
3. A convex lens of refractive index=1.5, covered at the convex surface behaves as a concave mirror of f=20cm. What is the radius of curvature of its curved surface?
4. Calculate the image distance for an object of height 12 mm at a distance of 0.20 m from a concave lens of focal length 0.30 m and state the nature and size of the image.
5. A biconvex lens made of a transparent medium of n=1.5 is immersed in a medium of n=1.33. Will the lens behave by converging or diverging?
6. Find the focal length of a convex mirror if its radius of curvature is 25 cm.
7. What is the difference between a convex mirror and a lens?
8. A concave mirror produces a 10 cm long image of an object of a height of 2 cm. What is the magnification produced?
9. Find the radius of curvature of a convex mirror with a focal length of 50 cm.
10. An object 1 cm high is held near a concave mirror of magnification 10. How tall will be the image?

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