Muskan Shafi Education Content Expert
Education Content Expert
Refraction by Spherical Lenses is the phenomenon of convergence or divergence of light rays by spherical lenses to form images. Refraction of light is the basic principle behind the design and working of lenses.
- Refraction is the bending of light when it travels from one medium to another.
- Lens is a transparent glass piece bounded by two spherical surfaces and is used to magnify objects.
- Spherical Lenses are a type of optical instrument with a curved surface that causes light rays to converge or diverge.
- Concave Lens and Convex Lens are the two types of spherical lenses that cause refraction of light.
Refraction by Spherical Lenses helps in the formation of images when light is refracted by a concave or convex lens.
Read More: NCERT Solutions for Class 10 Science Light- Reflection and Refraction
Key Terms: Refraction, Spherical Lenses, Convex Lens, Concave Lens, Centre of Curvature, Principal Axis, Virtual Image, Real Image
What is Refraction?
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Refraction is the change in the direction of light when it passes from one medium to another.
- It is the bending of light when it passes from one medium to another.
- Light rays are bent due to the difference in density between the two substances.
- Optical instruments such as magnifying glasses, lenses and prisms work on the principle of refraction.
- Refraction of light is the phenomenon which enables to focus the light on the retina.
Refraction
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What are Spherical Lenses?
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Spherical Lenses are optical lenses with curved surfaces that cause the light rays to converge or diverge. Spherical lenses are transparent pieces cut off from a bigger sphere. These lenses either converge or diverge light rays to form an image.
Spherical Lenses are divided into two major types which are:
- Concave Lens
- Convex Lens
Concave Lens
- Concave lens is a spherical lens whose reflecting surface is curved inwards.
- It is also known as a ‘Diverging Lens’ as they diverge or spread the beam of light in different directions.
- Concave lenses are used as corrective lenses for people having myopia or short-sightedness.
- They are used in spectacles, lasers, flashlights, cameras, etc.
- Concave lenses create a virtual image of the object.
- Concave lenses are thicker at the edges and thinner in the middle.
Concave Lens
Convex Lens
- Convex lens is an optical lens whose reflecting surface bulges outwards.
- It is known as a ‘Converging Lens’ as it converges the light rays at a point.
- They are used in eyeglasses, magnifying glasses, microscopes, cameras, etc.
- The image formed by convex lenses is virtual and erect.
- Convex lenses are thicker in the middle and thinner at the edges.
Convex Lens
Read More: Biconvex Lens
Difference Between Convex and Concave Lens
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Concave and Convex Lenses are the two major types of spherical lenses that are used in the refraction of light. The difference between concave and convex lenses is as follows:
| Concave Lens | Convex Lens |
|---|---|
| Concave lenses are thinner in the middle and thicker at the edges. | Convex lenses are thicker in the middle and thinner at the edges. |
| They are also known as ‘Diverging Lenses’. | They are also known as ‘Converging Lenses’. |
| They are used in glasses, telescopes, spy holes in doors, etc. | They are used in cameras, overhead projectors, telescopes, magnifying glasses, etc. |
| Concave lenses are used for the correction of myopia. | Convex lenses are used for the correction of Hypermetropia. |
| They have a negative focal length. | They have a positive focal length. |
| Concave lenses diverge the incident rays away from the principal axis. | Convex lenses converge the incident rays towards the principal axis. |
Read More: Difference Between Concave and Convex Mirror
Refraction and Spherical Lenses
Basic Terminologies Related to Spherical Lenses
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Refraction by Spherical Lenses deals with the phenomenon of the bending of light by concave and convex lenses. Here are a few important terms when dealing with the concept of Refraction by Spherical Lenses:
- Centre of Curvature: It is the centre of the hollow sphere of glass, from which the lens is derived.
- Principal Axis: It is the imaginary line joining the centres of curvatures of both spheres.
- Principal Focus: It is a point on the principal axis, where light rays parallel to the principal axis converge or appear to diverge. It is also known as the Focal Point.
- Optical Centre: It is a geometric centre of the curved lens or a point within the lens where the diameter of the lens and the principal axis intersect.
- Focal Length: It is the distance between the principal focus and the optical centre. It is symbolized by ‘f’.
Parts of Spherical Lenses
Rules for Image Formation by Lenses
Given below are a few guidelines that are followed during image formation by spherical lenses:
- When a ray strikes a concave or convex lens at its optical centre, then it continues to follow its path.
- When a ray that is parallel to the principal axis strikes a concave or convex lens, the reflected ray passes through the focus on the principal axis.
- When a ray that is passing through focus strikes a concave or convex lens, the reflected ray passes parallel to the principal axis.
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Refraction by Convex Lens
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The image formation by Convex Lens through refraction is as follows:
- Case 1: When the object is placed at infinity, a real image is formed at the focus. The image is highly diminished and is point-sized.
- Case 2: When the object is placed beyond the Centre of Curvature, the image is formed between the centre of curvature and focus. The image will be a real image and will be diminished in size.
- Case 3: When the object is placed at the centre of curvature, a real image is formed at the other centre of curvature which is of the same size as the object.
- Case 4: When the object is placed in between the centre of curvature and focus, a real image is formed behind the centre of curvature which is larger than that of the object.
- Case 5: When the object is placed at the focus, a real image is formed at infinity with a much larger size than that of the object.
- Case 6: When the object is placed in between the focus and the optical centre, a virtual image is formed with a larger size than the object.
Image Formation by Convex Lens
Here is a summary of the image formation by Convex lens:
| Object Location | Image Location | Nature of Image |
|---|---|---|
| Infinity | At F2 | Real, Inverted, Diminished, and Point-sized |
| Beyond 2F1 | Between 2F2 and F2 | Real, Inverted, and Diminished |
| Between 2F1 and F1 | Beyond 2F2 | Real, Inverted, and Enlarged |
| At F1 | At infinity | Real, Inverted, and Highly Enlarged |
| At 2F1 | At 2F2 | Real, Inverted, and Same Size |
| Between F1 and 0 | Same Side as the Object | Virtual, Erect, and Enlarged |
Read More: Difference Between Real and Virtual Images
Refraction by Concave Lens
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The image formation by concave lenses is as follows:
- Case 1: When the object is placed at infinity, a virtual image is formed at the focus which is highly diminished and point-sized.
- Case 2: When the object is placed at any finite distance, a virtual image is formed between the optical centre and the focus with a much smaller size than that of the object.
Image Formation by Concave Lens
Here is a summary of the image formation by concave lenses:
| Object Location | Image Location | Nature of Image |
|---|---|---|
| Infinity | At F1 | Virtual, Erect, and Highly Diminished |
| Beyond Infinity and Optical Centre | Between F1 and O | Virtual, Erect, and Diminished |
Read More: Light- Reflection and Refraction Important Questions
Things to Remember
- Spherical Lenses are transparent optical materials with two spherical surfaces to refract light rays.
- Spherical lenses are of two main types namely Concave Lens and Convex Lens.
- Concave Lenses are lenses with two spherical surfaces bulging outwards.
- Convex Lenses are lenses with two spherical surfaces bulging inwards.
- Refraction is the bending of light when it passes from one medium to another.
- Refraction of light by concave or convex lens leads to image formation.
- Concave lens forms virtual, erect, and diminished images.
- Convex lens forms both real and virtual images.
Sample Questions
Ques. What is a Lens? How is it different from Mirrors? (2 Marks)
Ans. A lens is a transparent piece that disperses or focuses light beams through the phenomenon of refraction of light. Concave and convex lenses are the main two types of lenses.
The main difference between a lens and a mirror is that lens refract (bend) the light rays while a mirror reflects or bounce back light rays.
Ques. What is the Sign Convention for distance in Image Formation by Spherical Lenses? (3 Marks)
Ans. The sign convention for distance in image formation by spherical lenses is as follows:
- Distance is measured through the optical centre of the lens.
- Distance measured in the same direction as that of the incident ray is taken as positive.
- Distance measured against the direction of the incident ray is considered negative.
- Distance measured upward and perpendicular to the principal axis is considered positive.
- Distance measured downward and perpendicular to the principal axis is considered negative.
Ques. What is Power of a Lens? (3 Marks)
Ans. Power of a Lens is a measure of the degree to which a lens can converge or diverge the light rays falling on it.
Power of Lens (P) = 1/ f
Here, f is the focal length of the lens measured in meters.
The SI unit of the Power of a Lens is Dioptre (D). A lens with a short focal length has more power compared to a lens with a long focal length.
Ques. Explain the properties of the image by Convex Lens when the object is placed in Focus. (2 Marks)
Ans. When the object is placed in Focus, one of the rays will move parallel to the principal axis, strike the lens, and then pass through focus. The other ray will pass through the centre and the two rays finally meet at infinity. The properties of the image formed in this case will be inverted, real, and magnified to a large extent.
Ques. What is Lens Formula? (3 Marks)
Ans. Lens Formula is an equation that gives the relationship between the distance of an object, the distance of an image, and the focal length of the lens.
Lens Formula is expressed as follows:
\( \frac{1}{v} + \frac{1}{u} = \frac{1}{f}\)
Here
- v is the distance of the image from the lens.
- u is the distance of the object from the lens.
- f is the focal length of the lens.
Ques. What is a Concave Lens? (2 Marks)
Ans. Concave lenses are a type of spherical lens that is formed by combining two spherical surfaces in such a manner that they are curved inwards. Concave lenses are also called diverging lenses since they diverge the light rays falling on them. These lenses are thicker at the edges and thinner in the middle.
Ques. What is the sign convention in spherical lenses? (3 Marks)
Ans. The sign conventions for spherical lenses are as follows:
| Distance | Positive Value (+) | Negative Value (-) |
|---|---|---|
| u | Real | Virtual |
| v | Real | Virtual |
| f | Convex Lens | Concave Lens |
Ques. What is Lateral Magnification? (3 Marks)
Ans. Lateral Magnification is the ratio of the size of the image to the size of the object. It is also known as linear magnification or transverse magnification.
Lateral Magnification is given as
m = v/u
If m is negative, the image will be inverted. If m is positive, the image will be upright.
Ques. List the rules for obtaining images formed by convex lenses. (3 Marks)
Ans. In a convex lens, an image is formed after refraction at a point where the two refracted light rays meet. The rules for the same are:
- The incident ray parallel to the principal axis passes through the focus of the lens after refraction.
- The incident ray that passes through the optical centre of the convex lens goes straight and does not bend after refraction. It passes along the path of the principal axis and does not deviate after refraction.
- The incident ray that passes through the focus of the convex lens becomes parallel to the principal axis after refraction.
Ques. Explain the image formation in a convex lens when the object is at infinity. (2 Marks)
Ans. When the object is at infinity, the rays will move parallel to the principal axis, strike the lens, and then converge to meet at the focus. The image thus formed will be a real image at the focus and its size will be a tiny or point image.
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