NEET Physics Notes Magnetostatics EMI and AC, EM waves-Ray Optics-Reflection of Light
Reflection of Light
According to the laws of reflection, the incident ray, reflected ray and the normal drawn on the reflecting surface at the point of incidence lie in the same plane, and the angle of incidence
Laws of reflection are true for reflection from a polished mirror or from an unpolished surface or for diffused reflection.
Whenever reflection takes place from a denser medium, the reflected rays undergo a phase change of n.
Reflection from a Plane Mirror
- If a ray is incident on a plane mirror at an angle of incidence i, then it suffers a deviation of π – 2i
- While keeping an object fixed, a plane mirror is rotated in its plane by an angle 6, then the reflected ray rotates in the same direction by an angle 2θ
- Focal length as well as the radius of curvature of a plane mirror is infinity. Power of a plane mirror is zero.
- If two plane mirrors are inclined to each other at an angle θ, the total number of images formed of an object kept between themwhen it is odd
- The minimum size of a plane mirror fixed on a wall of a room, so that a person at the centre of the room may see the full image of the wall behind him, should be the size of the wall.
Reflection from a Spherical Mirror
- A spherical mirror is a part of a hollow sphere whose one surface is polished, so that it becomes reflecting. The other surface of the mirror is made opaque.
- Images formed by a concave mirror may be real or virtual, may be inverted or erect, and may be smaller, larger or equal in size to that of the object. The image is virtual and erect when the object is placed between the pole and’the principal focus of concave mirror. In all other cases, the image formed is real and inverted one
- Image formed by a convex mirror is virtual, erect and diminished in size irrespective of the position of the object. Moreover, image is formed in between the pole and the principal focus of the mirror.
- The focal length of a spherical mirror is half of its radius of curvature, i.e
Let an object be placed at a distance u from the pole of a mirror and its’image is formed at a distance v from the pole. Then, according to mirror formula
The power of a mirror (in dioptre), is given as
If a thin object of height h is placed perpendicular to the principal axis of a mirror and the height of its image be h’, then the transverse or lateral magnification produced is given by
Negative sign of magnification means the inverted image and positive sign means an erect image.
When a small sized object is placed along the principal axis, then its longitudinal (or axial) magnification is given by
Refraction of Light
When light passes from one medium, say air, to another medium, say glass, a part is reflected back into the first medium and the rest passes into the second medium.
When it passes into the second medium, it either bends towards the normal or away from the normal. This phenomenon is known as refraction.
For a given pair of media, the ratio of the sine of angle of incidence to the sine of angle of refraction (r) is a constant, which is called the refractive index of second medium, w.r.t. first medium. Thus,
Refractive index is a unitless, dimensionless and a scalar quantity.
The refractive index of a medium w.r.t. vacuum (or free space) is known as its absolute refractive index. It is defined as the ratio of the speed of light in vacuum (c) to the speed of light in a given medium (v).
Value of absolute refractive index of a medium can be 1 or more than 1 but never less than 1.
When light travels from one material medium to another, the ratio of the speed of light in the first medium to that in the second medium is known as the relative refractive index of second medium, w.r.t. the first medium. Thus,
When light undergoing refraction through several media finally enters the first medium itself, then
When the object is in denser medium and the observer is in rarer medium, then real and apparent depth have . Real depth
Refraction from a Spherical Surface
Let an object be placed in a medium of refractive index at a distance u from the pole of a spherical surface of radius of curvature R and after refraction, its image formed in a medium of refractive index n2 at a distance v, then
The relation is true for all surfaces, whether the image formed is real or virtual.