NCERT Solutions for Class 10 Science Chapter 9 Light Reflection and Refraction

Properties of images formed by a plane mirrors:

1. The image formed by a plane mirror is virtual and erect.
2. It is of the same size as the object.
3. The image is formed as far behind the mirror as the object is in front of it i.e., u = v.
4. It is laterally inverted i.e., image is inverted sideways with respect to the object.

Properties of the image formed by a convex mirror:

• (i) This image is always virtual and erect.
• (ii) The image is highly diminished or point sized.
• (iii) It is always formed between F and P.
• (iv) As the object is moved towards the pole of convex mirror, image also moves towards its pole and gradually increases size till its size becomes almost equal to that of the object.

Laws of refraction of light: The refraction of light obeys the following two laws:
First law: The incident ray, the refracted ray and normal to the surface of separation at the point of incidence, all lie in the same plane.
Second law: The ratio of the sine of the angle of incidence and the sine of the angle of refraction is constant for a given pair of media. Mathematically,
[sin i] / [sin r] = n (a constant).
The ratio n2 is called refractive index of the second medium with respect to the first medium. The second law of refraction is also called Snell’s law of refraction.

Refraction of light: The phenomenon of bending of light from its straight line path as it passes obliquely from one transparent medium to another is called refraction of light.
The path of the ray of light in the first medium is called incident ray.
The path of the ray of light in the second medium is called refracted ray.
The angle between the incident ray and the normal at the surface of separation is called angle of incidence (i).
The angle between the refracted ray and the normal at the surface of separation is called angle of refraction (r).

Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

Radius of curvature, R = 20 cm Radius of curvature of a spherical mirror = 2 × Focal length (f) R = 2f f = r/2 = 20/2 = 10 cm Hence, the focal length of the given spherical mirror is 10 cm.

When an object is placed between the pole and the principal focus of a concave mirror, the image formed is virtual, erect, and enlarged.

Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of them. They are preferred as a rear-view mirror in vehicles because they give a wider field of view, which allows the driver to see most of the traffic behind him.

The light ray bends towards the normal. When a ray of light travels from an optically rarer medium to an optically denser medium, it gets bent towards the normal. Since water is optically denser than air, a ray of light travelling from air into the water will bend towards the normal.

Refractive index of a medium nm is related to the speed of light in that medium v by the relation: n = c/v Where, c is the speed of light in vacuum/air The refractive index of diamond is 2.42. This suggests that the speed of light in diamond will reduce by a factor 2.42 compared to its speed in air.

Power of lens is defined as the reciprocal of its focal length. If P is the power of a lens of focal length F in metres, then P = 1/f The S.I. unit of power of a lens is Dioptre. It is denoted by D. 1 dioptre is defined as the power of a lens of focal length 1 metre. 1 D = 1 per meter