10th Class Science Light - Reflection and Refraction Light- Reflection and Refraction

 Light Reflection and Refraction

• Light: It is a form of energy which when reflected by objects falls on the eyes and enables us to see the objects. Light propagates as electromagnetic waves. It does not require a medium for its propagation.

• The speed of light in free space or vacuum is\[3\times {{10}^{8}}\,m\,{{s}^{-1}}\].All colours of light travel with different speeds. The speed of light in a medium is less than that in vacuum.

• Reflection: The bouncing back of light rays incident on a surface in the opposite direction in the same medium is called reflection of light.

• Reversibility of light: If a ray of light is reversed, it always retraces its previous path. Object and image positions are interchangeable.

• Refraction: The bending of light rays when they pass from one optical medium to another is called refraction of light.

• Refractive index: The refractive index of a single medium is measured with respect to air or

   \[vacuum{{.}_{air}}{{\mu }_{medium}}=\frac{\sin i}{\sin r}\]

\[\text{The refractive index of a medium}\,{{\mu }_{m}}=\] \[\frac{\text{velocity of light in vacuum or air}}{\text{velocity of light in medium}}=\frac{c}{c'}\]

• Different colours travel with different velocities in a medium. Velocity of light is maximum for red and minimum for violet in a medium.

• All colours travel with the same velocity in vacuum (air).

• The refractive index is minimum for red and maximum for violet.

• When a light ray passes from one medium into another, the frequency remains constant but the velocity and wavelength change.

• Principal focus and focal length: Light can pass through a lens from either directions. Therefore, a lens has two principal foci which are situated at equal distances from the optical centre, one on either side of the lens. These are called first focal point \[{{F}_{1}}\] and the second focal point\[{{F}_{2}}\].The focal length of a lens is the distance between the optical centre and principal focus of the lens.

• Power of lens

The power of a lens is defined as the reciprocal of its focal length. The S.I. unit is diopter. The power of a converging lens is positive and that of a diverging lens is negative.

• Magnification

The linear magnification is defined as the ratio of the size of the image to the size of the object.

\[\therefore \]\[\text{Magnification}(M)=\frac{v}{u}=\frac{\text{height of the object}\,\text{(I)}}{\text{height of the}\,object\,(O)}\]