ICSE Class 8 Physics Notes Chapter 5 Light Energy

ICSE Physics Class 8 Notes Chapter 5 Light Energy

→ Light is a form of energy which produces in us the sensation of sight i.e. we can see objects only when light falls on them and then reflected into our eye.

→Velocity of light in air or in vacuum is 300000 km per second.
Or
3 x 10⁸ms^-1

→ As light passes into different mediums its speed changes and depends upon the density of medium i.e. it decreases with increase in density i.e. it is 2.25 x 10⁸ m/s in water and 2 x 10⁸ ms^-1 in glass as water is denser than air (\({ }_{w}^{a}µ\)= 1.33) and glass is still optically denser than water (\({ }_{g}^{a}µ\) = 1.5) i.e. slower in water and still slower in glass.

→ Light travels in a straight line.

→ As light travels from one transparent medium to other transparent medium and falls oblique at another medium, its path changes and this change in path is called REFRACTION OF LIGHT.

→ When ray of light travels from RARER (less-denser) to DENSER medium, it bends TOWARD the normal AND when it travels from a DENSER to a RARER medium it bends away from NORMAL.

→ ANGLE of INCIDENCE : “The angle which incident ray makes with normal”. “ ∠i ”
ANGLE OF REFRACTION : “The angle which refracted ray makes with normal” “ ∠r ”
∠i is not equal to ∠r

→ LAWS OF REFRACTION or SNELL’S LAWS OF REFRACTION :

1. Incident ray, normal at the point of incidence and Refracted ray all lie in the same plane.
2. Ratio of sine of angle of incidence to the sine of angle ofrefraction is constant.

= Constant = Refractive index = µ (mew)
R.I = µ =

→ EFFECTS OF REFRACTION :

1. A coin placed in water appears to be raised.
2. Swimming pool seen from above appears SHALOW.
3. A pencil in water appears to be bent.
4. MIRAGE in desert, EARLY Sunrise, LATE SUN set are all due to RFRACTION of light.

→ White light is a band of seven colours -VIBGYOR. Speed of all colours of the white light in AIR or VACUUM is same, but different different transparent mediums.

→ In glass or water Speed of VIOLET colour is MINIMUM and speed of RED light is MAXIMUM

→ Refractive index of medium is minimum for VIOLET light and R.I. of medium is maximum for red light.

→ DISPERSION : “Hie spliting (breaking) of white light into seven colours is called DISPERSION OF LIGHT.

→ CAUSE OF DISPERSION : Speed of different colours is different in glass or water and different colours get separated from each other on refraction at second surface of glass prism.

ICSE Class 8 Physics Notes