CBSE Class 12 Physics Optics Notes

CBSE Class 12 Physics Optics. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.




1. Reflection by convex and concave mirrors.

a.Mirror formula 1/v+1/ u =1/f  , where u is the object distance, v is the image distance and f is the focal length.

b.Magnification m=-v/u=f-v/f=f/f-u m is -ve for real images and +ve for virtual images.

2. Refraction

a.Ray of light bends when it enters from one medium to the other, having different optical densities.

b. Sun can be seen before actual sunrise and after actual sunset due to Atmospheric refraction

c. An object under water ( any medium ) appears to be raised due to refraction when observed inclined

n =Re al depth/apparent depth and

Shift in the position (apparent) of object is x=1(1-1/n) Where t is the actual depth of the medium 

d.Snell’s law states that for a given colour of light, the ratio of sine of the angle of incidence to sine of angle of refraction is a constant.


e. Absolute refractive index is the ratio between the velocities of light in vacuum to the velocity of light in medium. For air n=1. 


f. When a ray of light travels from denser to rarer medium and if the angle of incidence is greater than critical angle, the ray of light is reflected back to the denser medium. This phenomenon is called Total internal reflection.
g. Diamond has a high refractive index, resulting with a low critical angle (C=24.40). This promotes a multiple total internal reflection causing its brilliance and luster. Some examples of total internal reflection are formation of mirage and working of an optical fibre.

h. When light falls on a convex refracting surface, it bends and the relation between U, V and R is
given by  n2/V-n1
/u=n2-n1 /R

i. Lensmaker’s formula or thin lens formula is given by


j. For Convex Lens R1 +ve R2 –ve Concave lens R1-ve R2 +ve The way in which a lens behaves as converging or diverging depends upon the values of nL and nm.

When two lenses are kept in contact the equivalent focal length is given by

1/F=1/f1+1/ff1 & P=P1+P2


-d/f1f& P=P1+P2-dP1P2

When kept at d distance k.the lens formulais given by 1/v-1/u=1/f 

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