CBSE Class 10 Science Chapter 10 Notes Light Reflection and Refraction

Light Reflection and Refraction Class 10 Notes Understanding the Lesson

1. Light: It is a form of energy which produces the sensation of sight.

• Light exhibits dual nature i.e. wave as well as particle nature.
• It travels with a speed of 3 x 10⁸ m/s in vacuum. However speed is inversely proportional to optical density of the medium.

2. Reflection: When light falls on a surface, it bounces back to the medium. The phenomena is called reflection.

3. Beam: A beam is a bundle of rays, which originates from a common source and travels in the same direction.

4. Laws of Reflection:
Terminology

• Incident ray: Ray of light coming from a source towards the reflecting surface.
• Reflected ray: Ray of light which is reflected back by a reflection surface.
• Normal: Perpendicular drawn to the reflecting surface.
• Angle of incidence: The angle between incident ray and normal at the point of incidence.
• Angle of reflection: The angle between reflected ray and normal at the point of reflection

Laws:

• The angle of incidence is equal to the angle of reflection.
• The incident ray, the normal to the mirror at the point of incidence and reflected ray, all lie in the same plane.
  These laws of reflection are applicable to all types of reflecting surfaces including spherical surfaces.

5. Types of mirror
1. Plane mirror
2. Spherical mirror

• Concave mirror
• Convex mirror

Concave Mirror: A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of sphere, is called a concave mirror.
Convex Mirror: A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.

6. Basic terms of Spherical Mirrors:

• Centre of curvature: The centre of a hollow sphere of which the curved or spherical mirror forms a part is called centre of curvature.
• Radius of curvature (R): The radius of sphere of which the reflecting surface of a spherical mirror forms a part is called the radius of curvature of the mirror.
• Pole: The centre of the reflecting surface of spherical mirror. The pole is usually represented by the letter P.
• Principal axis: Its an imaginary line passing through the centre of curvature and pole.
• Aperture: The diameter of the reflecting surface of the spherical mirror is called its aperture.
• Principal focus: A point on the principal axis of a spherical mirror where the rays of light parallel to the principal axis meet or appear to meet after reflection from the spherical mirror is called principal focus.
• Focal length (f): The distance between the pole and principal focus (F) of a spherical mirror is called the focal length of the mirror. It is denoted by f.
• \(f=\frac{\mathrm{R}}{2}\)

7. Type of image

Real image	Virtual image
1. When rays of light after reflection meets at a point, real image is formed.	1. When rays of light do not actually meet but appear to meet at a point after reflection, virtual image is formed.
2. Real image can be obtained on screen.	2. Virtual image cannot be obtained on screen.
3. Real image is formed in front of mirror.	3. Virtual image is formed behind the mirror.
4. Real image is always inverted.	4. Virtual image is always erect.

Representation of images formed by spherical mirror using ray diagrams:

• In order to locate the image of an object, an arbitrarily large number of rays emanating from a point could be considered.
• The intersection of reflected ray gives the position of image.

8. Rules for obtaining image:
(i) A ray parallel to the principal axis, after reflection, will pass through the principal focus in case of concave mirror or appear to diverge from the principal focus in case of a convex mirror.

(ii) A ray passing through principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror after reflection will emerge parallel to the principal axis.

(iii) A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror after reflection, is reflected back along the same path.

(iv) A ray incident obliquely to the principal axis, towards point P (pole of the mirror) on the concave or convex mirror is reflected obliquely.

9. Formation of image by concave mirror
Image formation by a concave mirror for different positions of the object

S. No.	Position of the object	Position of the image	Size of the image	Nature of the image
(a)	At infinity	At the focus F	Highly diminished, point-sized	Real and inverted
(b)	Beyond C	Between F and C	Diminished	Real and inverted
(c)	At C	At bc	Same size	Real and inverted
(d)	Between C and F	Beyond C	Enlarged	Real and inverted
(e)	At F	At infinity	Highly enlarged	Real and inverted
(f)	Between P and F	Behind the mirror	Enlarged	Virtual and erect

Ray diagram for the image formation by a concave mirror

10. Formation of image by convex mirror

S. No.	Position of the object	Position of the image	Size of the image	Nature of the image
(a)	At infinity	At the focus F, behind the mirror	Highly diminished, point-sized	Virtual and erect
(b)	Between infinity and the pole P of the mirror	Between P and F, behind the mirror	Diminished	Virtual and erect

Ray diagram for image formation by convex mirror

11. Uses of mirrors
(a) Uses of concave mirrors:

• Concave mirrors are commonly used in torches, search lights and vehicles headlights to get powerful beam of light.
• It is used in shaving mirrors to see large image of the face.
• The dentists use concave mirror to see large images of the teeth of patients.
• Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

(b) Uses of convex mirrors:

• Convex mirrors are used as rear-view (wing) mirrors in vehicles.
• Convex mirrors are used as street reflectors because they are able to spread light over a bigger area.

12. Sign convention for reflection by spherical mirrors:

• The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
• All distances parallel to the principal axis are measured from the pole of the mirror.
• All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative.
• Distances measured perpendicular to and above the principal axis (along +y-axis) are taken as positive, (u)

13. Mirror formula and Magnification
Mirror formula
\(\frac{1}{u}+\frac{1}{v}=\frac{1}{f}\)

Refractive index:
Refractive index: The ratio of speed of light in vacuum (c) to the speed of light in any medium (v) is called refractive index of the medium.

Relative refractive index:
The relative refractive index of a medium with respect to other medium is the ratio of speed of light in the second

Here, n₂₁ = Relative refractive index of medium 1 with respect to medium 2.

14. Some applications of refraction:

• Bottom of a tank or a pond containing water appears to be raised due to refraction.
• When a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab.
• When a pencil is partly immersed in water, it appears to be displaced at the interface of air and water.
• A lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from sides.
  Lens: A transparent medium bound by two surfaces, of which one or both surfaces are spherical.

15. Convex lens: A lens may have two spherical surfaces, bulging outwards. Such a lens is called a double convex lens or convex lens.

• It is thicker at the middle as compared to the edges.
• Convex lens converges light as shown in Figure above.

Hence, convex lenses are called converging lens.

16. Concave lens: A double concave lens is bounded by two spherical surface curved inwards.

• It is thicker at the edges than in the middle.
• Concave lens diverges light and is called diverging lens.

17. Basic terms of spherical lens:

• Principal axis: A line joining the centre of curvatures of two spherical surfaces forming a lens is called principal axis. The line joining C₁ and C₂ is the principal axis (see figure below).
• Principal focus: A point on the principal axis of a lens where all rays of light parallel to the principal axis meet (figure a) or appears to meet (figure b) after passing through the lens is called principal focus of the lens.
• Optical centre: The central point of a lens (O) through which a ray of light pass undeviated is called optical
  
• Focal length: The distance between the principal focus and optical centre of a lens is called focal length of lens. It is denoted by f.
• Aperture of lens: The effective diameter of circular outline of a spherical lens is called its apperture.

18. Rules for making ray diagram

1. A ray of light from the object, parallel to the principal axis, after refraction from a lens passes through the principal focus or appears to diverge from the principal focus

2. A ray of light passing through the principal focus or appearing to meet at the principal focus after refraction, will emerge parallel to the principal axis.

3. A ray of light passing through the optical centre of lens will emerge without any deviation.

19. Image formation by convex lens. Nature, position and relative size of the image formed by a convex lens for various positions of the object.

S. No.	Position of the object	Position of the image	Relative size of the image	Nature of the image
(a)	At infinity	At focus F2	Highly diminished, point-sized	Real and inverted
(6)	Beyond 2F1	Between F2 and 2F2	Diminished	Real and inverted
(c)	At 2FX	At 2F2	Same size	Real and inverted
(d)	Between F1 and 2F1	Beyond 2F2	Enlarged	Real and inverted
(e)	At focus F1	At infinity	Infinitely large or highly enlarged	Real and inverted
(f)	Between focus F1 and optical centre 0	On the same side of the lens as the object	Enlarged	Virtual and erect

20. Ray diagram for the image formation by convex lens:

21. Image formation by concave lens:

Position of the objectPosition of the imageRelative size of the imageNature of the imageAt infinityAt focus F₁ Highly diminished, point-sizedVirtual and erectBetween infinity and optical 1 centre 0 of the lensBetween focus F₁ and optical centre 0DiminishedVirtual and erect

22. Sign convention of spherical lens:

• Sign conventions of lens is same as sign convention of mirrors
• The focal length of convex lens is positive and concave lens is taken as negative.

23. Lens formula and magnification:
\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)

u = object distance
v = image distance
f= focal length

24. Magnification (m)
Magnification is defined as the ratio of the height of image to the height of object.
\(m=\frac{\text { Height of the image }}{\text { Height of the object }}=\frac{h^{\prime}}{h}=\frac{v}{u}\)
h’ = height of image
h = height of object

25. Power of a lens
The power of a lens is defined as reciprocal of its focal length.
\(P=\frac{1}{f}\)
f = focal length (in metre)

• The SI unit of power is ‘dioptre’. It is denoted by the letter D.
• 1 dioptre is the power of a lens whose focal length is 1 metre, 1 D = 1 m^-1
• Power of convex lens is positive and concave lens is negative.

26. Combination of lens
\(\frac{1}{f_{\text {net}}}=\frac{1}{f_{1}}+\frac{1}{f_{2}}\)
f_net ⁼ Net focal length f_x = focal length of lens 1 f₂ = focal length of lens 2
P net = Power of combination
P₁ = Power of lens 1
P₂ = Power of lens 2.

Class 10 Science Chapter 10 Notes Important Terms

Reflection: When light falls on a surface and bounces back to the medium, the phenomenon is called reflection.

Concave mirror: A spherical mirror, whose reflecting surface is curved inwards.

Convex mirror: A spherical mirror, whose reflecting surface is curved outwards.

Magnification: Magnification is expressed as a ratio of the height of image to the height of object.

Refraction: The deviation of light rays from its path when it travels from one transparent medium to another transparent medium is called refraction of light.

Lens: A transparent medium bound by two surfaces, of which one or both surfaces are spherical, forms a lens.