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What is the difference between a concave lens and a convex lens?

Class 8 ScienceNCERT chapter: Light: Mirrors and Lensesइसे हिंदी में पढ़ें

Exam-ready answer

A convex lens is thicker in the middle than at the edges. It makes parallel rays meet at a point, so it is a converging lens. A concave lens is thinner in the middle than at the edges. It spreads parallel rays apart, so it is a diverging lens. Only a convex lens can form real images.

Explanation

A lens is a piece of transparent material, such as glass, with at least one curved surface. Light passing through a lens bends, that is, it gets refracted. The two kinds of lens can be told apart by touch: a convex lens feels thicker in the middle and thinner at the edges, while a concave lens feels thinner in the middle and thicker at the edges.

When sunlight falls on a convex lens, the rays come together at a point beyond the lens, forming a small, bright spot on a sheet of paper. This shows that a convex lens converges light. A convex lens can form a real, inverted image of a distant object on a screen, and when an object is held close to it, the lens shows an enlarged, upright image. That is why it is used as a magnifying glass.

A concave lens spreads parallel rays of light apart, so they never meet at a point on the other side. It always forms an upright image that is smaller than the object, and this image cannot be obtained on a screen. Convex lenses are used in magnifying glasses, cameras and microscopes, while concave lenses are used in the spectacles of people who cannot see distant objects clearly.

Points to remember

  1. Lens: a transparent material with at least one curved surface that refracts (bends) light.
  2. Convex lens: thicker in the middle, converges parallel rays; also called a converging lens.
  3. Concave lens: thinner in the middle, diverges parallel rays; also called a diverging lens.
  4. A convex lens can form real, inverted images on a screen, and an enlarged, upright image when the object is close.
  5. A concave lens always forms an upright, diminished image that cannot be obtained on a screen.

Exam tip

Present the answer as a comparison table: shape, effect on parallel rays, nature of image and uses. Draw two ray diagrams with arrows, one showing rays meeting after a convex lens and one showing rays spreading after a concave lens.

Questions students also ask

How can you identify a convex lens without touching it?
Look at small print through the lens held close to it. A convex lens makes the letters look bigger, while a concave lens makes them look smaller. A convex lens can also focus sunlight to a bright spot on paper.
Why should we never look at the Sun through a lens?
A convex lens converges sunlight into a very bright, hot spot. Looking at the Sun through it, or even directly, can seriously damage the eyes. The same focusing can burn paper, so lenses should be handled carefully in sunlight.
Why is a convex lens used as a magnifying glass?
When an object is placed close to a convex lens, the lens forms an enlarged and upright image of it. This makes small details, such as fine print or the parts of a flower, look bigger and easier to see.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.