Class 10 Science Chapter 9 Important Questions

Light is a form of electromagnetic radiation (radiant energy) That causes the sensation of sight. In vacuum/air light travels with a constant speed of 3 × 10⁸ m/s. Ordinarily light travels in straight line but truly speaking light exhibits wave character. Wave lengths of visible light vary from about 400nm for violet light to about 700nm for red light.
Properties of light are:
(i) Light is an electromagnetic wave so it does not require any medium for travel.
(ii) Light castes shadow of every object.
(iii)Light tends to travel in a straight line.
(iv) When light falls on a surface Reflection, Refraction and absorption may happen.

The straight path along which a light wave travels is called ray of light. A bundle of ray is called beam of light.

When light falls on a surface separating two media, a part of light is reflected, a part is refracted and a part is absorbed. A highly polished surface (mirror) mostly reflects the light falling on it. A transparent medium example glass, water etc., refracts most of the light falling on it. When a numbers of rays, starting from a point after refraction or reflection or refraction meet or appear to meet at another, point the second point is called the image of the first point.

The image formed is real if the reflected or refracted actually meet at a point. The image is virtual is the rays do not actually meet, but appear to meet at a point when produced backwards. A real image can be obtaining on the screen but a virtual image can be obtaining on a screen.

The basic laws of reflection of light:
(i) The angle of incidence is (i) is always equal to the angle of reflection (r).
(ii) The incident ray, the normal to the reflecting surface at the point of incidence, and the reflected ray all lie in the same plane.

Image of an extended object formed by a plane mirror has the following characteristic:
(i) The image formed is virtual and erect.
(ii) The image is of same size as the object.
(iii) The image is formed as far behind the mirror as the object is in front of it.
(iv) The images are laterally inverted.

A spherical mirror is that whose reflecting surface is spherical.
There are two type of spherical mirror.
(i) Concave mirror.
(ii) Convex mirror.
Properties of concave mirror are:
(a) A spherical mirror whose reflecting surface is curved inward is called a concave mirror.
(b) It is also known as the converging mirror.
Properties of convex mirror are:
(a) A spherical mirror whose reflecting surface is curved outward is called a convex mirror.
(b) It is also known as the diverging mirror.

The center of the reflecting surface of a mirror is called its pole (P).
The center of the sphere, the given mirror forms a part of which is called the center of curvature of the spherical mirror. Center of the curvature of a mirror lies outside its reflecting surface. For a concave mirror the center of curvature of a mirror lies in front of it but for a convex mirror it lies behind the mirror. The distance from pole to center of curvature is known as the radius of curvature of given mirror.

A straight line passing through the pole and the center of curvature of a spherical mirror is called its principal axis. Principal focus of a spherical mirror is a point on its principal axis where lights rays coming parallel to its principal axis after reflection actually converge (in case of a concave mirror) or appear to diverge from (in case of concave mirror) or appear to diverge from (in case of a convex mirror) Distance from pole to principal focus is called focal length (f) of given mirror.

Focal length of spherical mirror is half of its radius of curvature. Thus , f=R/2 or R=2f.

The position of the image formed by the spherical mirror can be found by considering any two of the following rays:
(i) The ray incident parallel to the principal axis, after reflection, passes through the principal focus of a concave mirror or appear to pass through the principal focus of a convex mirror.
(ii) A ray passing through the principal focus of a concave mirror or a ray directed toward the principal focus of a convex mirror is reflected parallel to the principal axis of the mirror.
(iii) A ray passing through the center of curvature in a concave mirror or a ray directed toward the center of curvature in a convex mirror, after reflection retraces its path.
(iv) A ray incident obliquely to the principal axis at the pole point is reflected oblique so that angle subtended by the two rays from principal axis are equal and in mutually opposite direction.

What is the nature and the position of the image when an object is placed at infinity in case convex mirror?

While considering reflection from spherical(curved) mirror we follow the new Cartesian Sign Convention. According to this convention:
(i) The object is taken on the left of the mirror, i.e. the incident ray strikes the mirror from left hand side.
(ii) All the distance parallel to the principal axis are measured from the pole of the mirror.
(iii) Distances in the direction of the incident light are taken positive and in opposite direction negative. In other word distances right to the pole are taken positive and the distances left to the pole negative.
(iv) The height measured downwards (i.e. Above the principal axis) are taken positive and the height measured downward (below the principal axis) are taken negative.

Mirror formula is a relation between the object distance (u), the image distance (v) and the focal length (f) (or radius of curvature R) of the mirror. According to mirror formula:
1/v + 1/u = 1/f or 1/v + 1/u = 2/R
The mirror formula is true for concave and convex mirror both, irrespective of the fact whether the image formed is real or virtual. However proper sign for u, v, f and R be applied as per Sign convention followed.

Magnification (m) of a spherical mirror is define as the ratio of the height of the image to the height of the object. It is found that –
Magnification (m) = (Height of image (h^1 ))/(Height of the Object (h))= -v/u
Magnification for real and inverted image is negative. Magnification for a virtual and erect image is positive.

Two basic laws of refraction of light:
(i) The incident ray, the refracted ray and the normal to the separating surface at the point of incidence, all lie in same plane.
(ii) The ratio of sign of angle of incidence (i) to the since of angle of refraction (r) is a constant. It is known as Snell’s law. This according to Snell’s law
(Sin i)/(Sin r) =a constant =n₂₁
Here n₂₁ is known as the relative refraction index of the second medium with respect to first medium.

When a light ray is refracted from medium number 1 into medium 1 into medium number 2, the relative refractive index of medium 2 w.r.t medium 1(n₂₁) is given by:
n₂₁ = (Speed of light in medium 1 (v₁))/(Speed of light in medium 2 (v₂))
Similarly, the refractive index of medium 1 with respect to medium 2 is given by:
n₂₁ = (Speed of light in medium 2 )/(Speed of light in medium 1 )= (v₂ )/v₁
therefore we conclude that n₂₁ × n₁₂ = 1
or n₁₂ = 1/n₂₁

Absolute refractive index n of a given medium is defined as:
n= (Speed of light in vaccume or air (c))/(Speed of light in given medium (v))
Refractive index is a unit less term. More over absolute refractive index of a medium has a value one or greater than one.
It is found that
n₂₁ = (v₁ )/v₂ = (n₂ (absolute refractive insex of medium 2) )/(n₁(absolute refractive index of medium 1))
For a given wavelength and pair of media n₂₁ is a constant whose value depend only upon the optical properties of the two media and is independent of the angle of incidence.

A medium with larger refractive index is called optically denser medium and the medium with lesser refractive index is called optically rarer medium. A ray of light travel from a rarer medium to denser medium bends toward the normal (i.e., Lr<Li). However a ray of light travelling from a denser medium to a rarer medium bend away from the normal (i.e. Lr>Li). Refractive index of air is generally taken as 1. Refractive index of water w.r.t. air is 1.33 and that of crown glass w.r.t air is 1.52. Refractive index of diamond is maximum equal to 2.42.

As a lens has two surface each surface has its own center of curvature. A straight line passing through the two center of curvature. A straight line passing through the two center of curvature of a lens is called its principal axis.

The effective diameter of the circular outline of a spherical lens is called its “aperture”.
For a thin lens aperture of lens is generally much less than its radius of curvature.

The position of the image formed by a lens can be found by a lens can be found by considering any two of the following rays:
(i) A ray incident parallel to the principal axis, after refraction, passes through the principal focus on other side of lens in a convex lens or appear to diverge from the principal focus located on the same side of a concave lens.
(ii) A ray of light passing through the principal focus in a convex lens or appearing to meet at the principal focus in a concave lens emerges parallel to the principal axis after refraction.
(iii) A ray of light passing through the optical center of lens emerges without suffering any deviation after refraction.

In dealing with lenses we use the same Cartesian Sign Convention as followed for mirror expect that now all the distances are measured from the optical center of the lens. Lens formula is a relation correlating u, v and f for a lens. According to lens formula we have:
1/v – 1/u = 1/f
For a real object a concave lens always from a virtual, erect and diminished image. For an object situated at infinity the point sized image is formed at the principal focus F1. In all other cases the diminished image is formed on the same side of the lens, as the object, between optical center O and principal focus F1.

Lens formula is a relation correlating u, v and f for a lens.
According to lens formula we have
1/v – 1/u = 1/f
The relation is true for all lenses and all types of images formed.
However, appropriate sign convention must be followed for u, v and f.

The magnification (m) produced by a lens is given by
Magnification (m) = (Height of image (h^1 ))/(Height of the Object (h))= v/u
Magnification of a lens is –ve for a real and inverted image but +ve for a virtual and erect image.