Class 8th Science Chapter 10 Lenses

Class 8th Science Chapter 10 Lenses

Class 8th Science Chapter 10 Lenses. A lens is a trans missive optical device that focuses or scatters a beam of light by refraction. A simple lens consists of a single piece of transparent material, while a composite lens consists of several simple lenses (elements), generally arranged along a common axis. Class 8th Science Chapter 10 Lenses

Class 8th Science Chapter 10 Lenses
Class 8th Science Chapter 10 Lenses

Class 8th Science Chapter 10 Lenses

Class 8th Science Chapter 10 Lenses. Lenses are made from materials such as glass or plastic, and are ground and polished or molded to the desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or scatter waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.

Class 8th Science Chapter 10 Lenses
Class 8th Science Chapter 10 Lenses

Class 8th Science Chapter 10 Lenses

Class 8th Science Chapter 10 Lenses. Lenses are used in various imaging devices such as telescopes, binoculars, and cameras. They are also used as visual aids in glasses to correct vision defects such as nearsightedness and farsightedness.

UNIT:10

word image 28

word image 29

Q1. Describe the paths of three rays passing through convex lens with the help of ray diagrams.

Ans: 1). Ray passing parallel to the principal axis will refract through the lens and travel through the focal point on the opposite side of the lens.

20190625_131207.jpg

2). A ray passing through the focal point will refract through the lens and travel parallel to the principal axis.

20190625_131135.jpg

3). A ray passing through the optical center will go unrefract.

20190625_131102.jpg

Q2. How focal length is affected when the lens of an eye becomes thicker?

Ans: The focal length is reduced when the lens of an eye becomes thicker. Parallel rays are focused in front of the retina.

Q3. Draw ray diagrams to show how a converging lens forms images. What are the characteristics of these images?

Ans. Characteristics of a converging lens images

  1. The object beyond 2F. The image formed between F and 2F on the other side of the lens is real inverted and smaller in size

20190626_151631.jpg

  1. The object on 2F. The image formed on 2F on the other side of the lens is real inverted and of the same size.

20190626_151537.jpg

  1. The object between F and 2F. The image formed beyond 2F on the other side of the lens is real inverted and larger in size.

20190626_151500.jpg

  1. The object on F. The image is formed at infinity on the other side of the lens. Parallel rays cannot show the image in diagram.

20190626_151417.jpg

  1. The object between F and O. A virtual image is formed on the same side of the lens at the point where the rays meet when extended back.

20190626_151336.jpg

Q4. Define the following terms related to a lens along with the diagram.

Principle axis: A line passing through center of curvature, optical center, and focal point is called principle axis or optical axis.

Optical center: The mid or central point of a lens is called its optical center.

Principal focus: The point where light rays meet after refraction through lens is called principle focus.

Focal length: The distance between optical center and principle focus is called focal length.

Q5. Write only two uses of concave and convex lens.

Ans. Uses of concave lenses

  1. They are used to correct short sightedness.
  2. They are used in flashlights to widen the beam of bulbs.

Uses of convex lenses

  1. They are used to correct long sightedness.
  2. They are used in cameras to focus light for a clean picture.

word image 30

Q1. Describe the features of a human eye that allows it to see.

Ans. Features of a human eye are the following.

Convex lens in the eye forms a small inverted image of things.

Iris controls the pupil. Pupil controls the amount of light.

Cornea and the watery fluid do the focusing of the rays of light.

Small inverted image is formed on the retina of the eye.

Optic nerve carries image from the retina to the brain.

Q2. Describe how the eye and camera are similar and pint out some differences between them.

Ans. Similarities

They are optical instruments. They can adjust the amount of light entering.

They have a convex lens at one end and a light sensitive film on the other side.

Their lenses form a real, small and inverted image of an object.

Differences

Human eye is an organic entity but a camera is a mechanical device.

A camera lens moves closer or further but a cornea in eye changes its shape.

Human eye has optic nerve a camera has no optic nerve.

Camera is uniformly sensitive to light but an eye has different sensitivity to light.

Q3. Write a note on short sightedness and long sightedness and also explain how to remove these defects using lenses.

Ans: Short sightedness

A person suffering from short sightedness cannot see far off objects clearly. Lens becomes thick and the focal length is reduced. Parallel rays are focused in front of the retina.

Correction: Concave lenses of suitable focal length are used to focus rays on the retina.

Long sightedness

A person suffering from long sightedness cannot see the near objects clearly. The lens becomes thinner and parallel rays are focused behind the retina.

Correction: Convex lens of suitable focal length are used to focus rays on the retina.

1 thought on “Class 8th Science Chapter 10 Lenses”

  1. Pingback: Class 8th Science Chapter 11 Electricity in Action - Daraz 24

Leave a Reply

Scroll to Top
%d bloggers like this: