Revision: Light – Reflection and Refraction Science English Medium Class 10 CBSE

The phenomenon due to which a parallel beam of light traveling through a certain medium, on striking some polished surface, bounces off from it, as a parallel beam, in some other direction, is called regular reflection.

The bouncing of light by any smooth or polished surface is called.

The principal axis is the straight line passing through the pole and the centre of curvature.

Aperture is the distance between the extreme points on the periphery of the mirror.

 Centre of curvature is the centre of the imaginary sphere to which the mirror belongs.

Principal focus of a spherical mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror, converge to or appear to diverge from.

“A mirror which is made from a part of a hollow sphere is called Spherical Mirror.

“A mirror made by silvering the inner surface such that reflection takes place from the bulging surface” is called Convex Mirror.
The Centre of curvature is towards the silvered surface.

Pole “is the mid-point of the mirror”.

The centre of a hollow sphere of which the mirror forms a part is called the centre of curvature.

An imaginary line passing through the pole and the centre of curvature of a spherical mirror is called principal axis.

It is a point on the principal axis, where a beam of light, parallel to the principal axis, after reflection actually meet.

The linear distance between the pole and the center of curvature is called the radius of curvature.

The linear distance between the pole and the principal focus is called focal length.

The focus of a concave mirror is a point on the principal axis of the mirror, where all the rays travelling parallel to the principal axis and close to it after reflection from the mirror converge to that point.

Normal to the surface of a mirror at any point is the straight line at the right angle to the tangent drawn at that point.

“A mirror made by silvering the outer or the bulging surface such that the reflection takes place from the concave surface.” Centre of curvature is towards the reflecting surface.

The distance between the pole and the principal focus is called the focal length (f) of a spherical mirror.

Pole is the centre of the reflecting surface, in this case, a spherical mirror.

Mirrors whose reflecting surfaces are spherical are called spherical mirrors.

OR

A spherical mirror is a part of a hollow sphere, whose one side is silvered and coated with red oxide and the other side is the reflecting surface.

The centre of the reflecting surface of a spherical mirror is a point called the pole. The pole is usually represented by the letter P.

OR

The central point of the reflecting surface of the mirror is called the 'pole' of the mirror.

A spherical mirror, whose reflecting surface is curved inwards, that is, faces towards the centre of the sphere, is called a concave mirror.

OR

A concave mirror is one whose reflecting surface is towards the centre of the sphere of which the mirror is a part.

A spherical mirror whose reflecting surface is curved outwards, is called a convex mirror.

OR

A convex mirror is one whose reflecting surface is away from the centre of the sphere of which the mirror is a part.

The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a centre. This point is called the centre of curvature of the spherical mirror. It is represented by the letter C.

OR

The centre of the sphere of which the mirror forms a part, is called the ‘centre of curvature' of the mirror.

The radius of the sphere of which the reflecting surface of a spherical mirror forms a part is called the radius of curvature of the mirror. It is represented by the letter R.

OR

The radius of the sphere of which the mirror forms a part, is called the 'radius of curvature' of the mirror.

A straight line passing through the pole and the centre of curvature of a spherical mirror. This line is called the principal axis.

OR

The straight line joining the pole and the centre of curvature of the mirror and extended on both sides is called the 'principal axis' of the mirror.

The distance of the image from the pole of the mirror is called the image distance (v).

The distance of the principal focus from the pole is called the focal length (f).

In a spherical mirror, the distance of the object from its pole is called the object distance (u).

When travelling obliquely from one medium to another, the direction of propagation of light in the second medium changes. This phenomenon is known as refraction of light.

OR

Light changes its direction when going from one transparent medium to another transparent medium. This is called the refraction of light.

OR

The bending of the light ray from its path in passing from one medium to the other medium is called 'refraction' of light.

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.

Refracted light is the part of light enters into the other medium and travels in a straight path but in a direction different from its initial direction and is called the refracted light.

The change in the direction of the path of light when it passes from one transparent medium to another transparent medium is called refraction. The refraction of light is essentially a surface phenomenon.

The refractive index of second medium with respect to the first medium is defined as the ratio of the sine of the angle of incidence in the first medium to the sine of the angle of refraction in the second medium.

An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.

"A concave lens is a lens bounded by two spherical surfaces curved inward (double concave), which is thicker at the edges than at the middle, and diverges parallel light rays passing through it."

The central point of a lens is its optical centre. It is usually represented by the letter O.

"A lens is a transparent material bounded by two surfaces, at least one of which is spherical, that refracts light to form an image."

"A convex lens is a lens that has two spherical surfaces bulging outward (double convex), and it converges light rays passing through it."

Power of a lens is defined as the ability of a lens to bend the rays of light. It is given by the reciprocal of focal length in metre.

The power of a lens is a measure of the deviation produced by it in the path of rays refracted through it.

The deviation of the incident light rays produced by a lens on refraction through it, is a measure of its power.

or

The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P.

OR

The power (P) of a thin lens is equal to the reciprocal of its focal length (f) measured in metres.

\[\frac {1}{v}\] + \[\frac {1}{u}\] = \[\frac {1}{f}\]

Magnification (m) = \[\frac{\text{Height of the image (}h'\text{)}}{\text{Height of the object (}h\text{)}}\] = \[\frac {h'}{h}\]

Magnification in terms of object and image distances:

Magnification (m) = \[\frac {h'}{h}\] = -\[\frac {v}{u}\]

The refractive index of medium 2 w.r.to medium 1:

\[n_{21} = \frac{\text{Speed of light in medium 1}}{\text{Speed of light in medium 2}} = \frac{v_1}{v_2}\]

The refractive index of medium 1 w.r.to medium 2:

\[n_{12}=\frac{\text{Speed of light in medium 2}}{\text{Speed of light in medium 1}}=\frac{v_{2}}{v_{1}}\]

The refractive index of the medium n_m  is given by:

\[n_{m} = \frac{\text{Speed of light in air}}{\text{Speed of light in the medium}} = \frac{c}{v}\]

OR

\[\frac {\text{sin i}}{\text{sin r}}\] = constant = n

n is called the refractive index of the second medium with respect to the first medium.

Magnification (m) = \[\frac{\text{Height of the Image}}{\text{Height of the object}}=\frac{h^{\prime}}{h}\]

Magnification in terms of object and image distances:

Magnification (m ) = \[\frac {h'}{h}\] = \[\frac {v}{u}\]

\[\frac {1}{v}\] - \[\frac {1}{u}\] = \[\frac {1}{f}\]

Power of lens (in D) = \[\frac{1}{\text{focal length (in metre)}}\]

or

P = \[\frac {1}{f}\]

or

P = \[\frac {1}{f (m)}\]

Power of a Lens in a Medium:

P = (n₂ - n₁)\[\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)\] = \[\frac {n_1}{f}\]

• The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence all lie in the same plane.
• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction. (This is true for angle 0 < i < 90°)
  If i is the angle of incidence and r is the angle of refraction, then,
  \[\frac {\text{sin i}}{\text{sin r}}\] = constant

This constant value is called the refractive index of the second medium with respect to the first.

• The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
• The ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is constant for the pair of given media. i.e., mathematically

\[\frac {\text{sin i}}{\text{sin r}}\] = constant = n

Where:

• i = angle of incidence
• r = angle of refraction
• n = refractive index of the second medium with respect to the first

This ratio is constant for a given pair of media and is known as the refractive index.

Note: If a light ray is incident along the normal (i = 0^∘), it passes undeviated into the second medium (r = 0^∘).

• Reflection occurs when light bounces off a smooth surface like a mirror, following fixed laws.
• Plane mirrors always form virtual, erect, and same-sized images that are laterally inverted.
• Curved surfaces (like a spoon) act as spherical mirrors, changing the image size and orientation depending on the object's position.

• 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.
• Distances measured perpendicular to and below the principal axis (along the y-axis) are taken as negative.

• Ray Diagrams help us find the nature, position, and size of images formed by lenses.
• A Parallel Ray passes through the focus in a convex lens and appears to diverge from the focus in a concave lens.
• Focus and Optical Centre Rays: A ray through the focus comes out parallel; a ray through the optical centre goes undeviated.