NEET Important Chapter - Ray Optics and Optical Instruments

Name of the Concept

Key Points of the Concept

1. Reflection of Light

• Reflection of Light is the phenomenon of bouncing back of light after striking a smooth surface. 

• According to laws of reflection, the incident ray, reflected ray and the normal ray, all lie in the same plane.

• Also, during reflection, angle of incidence is equal to the angle of reflection. 

2. Reflection from a spherical mirror

• Spherical mirrors are the part of a sphere whose one surface is polished to make it reflecting.

• It is majorly divided into two types: concave and convex mirrors.

• The image formed by a concave mirror is virtual and erect when the object is kept between the Pole and Focus of the mirror and for every other case, the image formed will be real and inverted. 

• The image formed by a convex mirror is always virtual, erect and diminished, no matter where the object is kept.

• Focal length of the concave mirror is the half of the radius of curvature of the spherical mirror i.e. $f= \dfrac{R}{2}$

3. Mirror formula

• Let the distance of the object from the pole be u, the distance of the image from the pole be v and focal length of the mirror be f. Then, 

$\dfrac{1}{f}= \dfrac{1}{v}+\dfrac{1}{u}$

• Magnification of the object of height, h ket perpendicular to the principal axis and image height, h’ is given by:
  $m = \dfrac{h}{h'} =-\dfrac{v}{u}= \dfrac{f}{f-u}=\dfrac{f-v}{f}$

• When the object is kept along the principal axis, the axial magnification (m’) is given by

$m’= \lgroup\dfrac{v}{u}\rgroup^{2} = \lgroup\dfrac{f}{f-u}\rgroup^{2}= \lgroup\dfrac{f-v}{f}\rgroup^{2}$

4. Refraction of Light

• Refraction of light is the phenomenon of bending of the light ray after passing from one medium to another due to the change in velocity of light in different media. 

• Light rays bend away from the normal while passing from a denser medium to a rarer medium and bend towards the normal while passing from a rarer medium to a denser medium. 

5. Laws of refraction

• The ratio of angle of incidence (i) to the sign of angle of refraction (r) is a constant, which is known as the refractive index of second medium w.r.t. the first medium. 

• $\dfrac{\sin i}{\sin r}= \dfrac{n_2}{n_1} = n_{21}$

This is also called Snell’s law. 

• Absolute refractive index (n) is defined as the ratio of speed of light in vacuum (c) to the speed of light in a medium (v).

Therefore, $n = \dfrac{c}{v}$

• When the object is in the denser medium and observer is in the rarer medium, the relationship between apparent depth and real depth is given by

$n_{12}= \dfrac{\text{Real depth}}{\text{Apparent depth}}$

6. Power of a lens

• Power of a lens is the reciprocal of focal length of the lens.

7. Refraction from a Spherical Surface

• Let an object be placed in a medium with refractive index $n_{1}$ at distance u from the spherical surface. An image is in a medium with refractive index $n_{2}$ at a distance of v from the spherical surface. 

$\dfrac{n_2}{v}-\dfrac{n_1}{u} = \dfrac{n_2-n_1}{R}$

8. Lens 

• Lens is the refracting medium bound by 2 surfaces

• The lens formula is given by:

$\dfrac{1}{f}= \dfrac{1}{v}-\dfrac{1}{u}$

• Magnification of the object of height, h ket perpendicular to the principal axis and image height, h’ is given by:
  $m = \dfrac{h}{h'} =\dfrac{v}{u}= \dfrac{f}{f+u}=\dfrac{f-v}{f}$

• When the object is kept along the principal axis, the axial magnification (m’) is given by

$m’= \lgroup\dfrac{v}{u}\rgroup^{2} = \lgroup\dfrac{f}{f+u}\rgroup^{2}= \lgroup\dfrac{f-v}{f}\rgroup^{2}$

9. Total Internal Reflection

• When a light ray goes from denser medium to rarer medium, it bends away from the normal. For a certain angle of incidence i.e. angle of incidence ($i_{c}$) known as critical angle, the angle of refraction in the rarer medium becomes 90°. 

• So, when the angle of incidence is greater than the critical angle, the light rays start reflecting back as shown in the figure below:

10. Prism

• A prism is homogeneous, transparent medium bounded by two plane surfaces at an angles of A to each other

11. Scattering of light

• Molecules of medium after absorbing the incoming radiations emit the radiations in all the directions. This is called the scattering of light. 

12. Optical Instruments

• An optical instrument is used to enhance and analyse the light waves

13. Simple Microscope

• It consists of a convex lens of small focal length and forms a magnified image when the object is placed between the optical centre and pole of the lens. 

14. Compound Microscope

• It is formed using two convex lenses. One is known as the eyepiece and the other is called the objective lens. 

• Focal length and aperture of the objective lens are smaller than that of the eyepiece. 

• The image formed by the objective lens is real, inverted and magnified, which acts as the object for the eyepiece and the final image is highly magnified w.r.t the original object. 

15. Telescope

• Telescope is an optical instrument which increases the visual angle of the eye by forming the image of distant objects at the least distance of distinct vision. 

16. Resolving power of optical Instruments

• It is the ability of an optical instrument to produce distinct images of two points of an object placed very close together. 

SI. No

Name of the Concept

Formula

1.

Mirror Formula

$\dfrac{1}{f}= \dfrac{1}{v}+\dfrac{1}{u}$

2.

Magnification Produced by Spherical Mirrors

$m = \dfrac{h}{h'} =-\dfrac{v}{u}= \dfrac{f}{f-u}=\dfrac{f-v}{f}$

3.

Axial Magnification Produced by Spherical Mirrors

$m’= \lgroup\dfrac{v}{u}\rgroup^{2} = \lgroup\dfrac{f}{f-u}\rgroup^{2}= \lgroup\dfrac{f-v}{f}\rgroup^{2}$

4.

Snell’s Law

$\dfrac{\sin i}{\sin r}= \dfrac{n_2}{n_1} = n_{21}$

5.

Power of Lens

$P=\dfrac{1}{f}$

6.

Refraction from a Spherical Surface

$\dfrac{n_2}{v}-\dfrac{n_1}{u} = \dfrac{n_2-n_1}{R}$

7.

Lens Formula

$\dfrac{1}{f}= \dfrac{1}{v}-\dfrac{1}{u}$

8. 

Magnification Produced by Lens

$m = \dfrac{h}{h'} =\dfrac{v}{u}= \dfrac{f}{f+u}=\dfrac{f-v}{f}$

9. 

Axial Magnification Produced by Lens

$m’= \lgroup\dfrac{v}{u}\rgroup^{2} = \lgroup\dfrac{f}{f+u}\rgroup^{2}= \lgroup\dfrac{f-v}{f}\rgroup^{2}$

10.

Lens Maker’s Formula

$P= \dfrac{1}{f} = (n_{21}-1)\lgroup\dfrac{1}{R_1}-\dfrac{1}{R_2}\rgroup$

11.

Total Internal Reflection

$\sin i_c = \dfrac{1}{n_{12}}$

12. 

Deviation Produced by Prism

$\delta = i+e-A$

For maximum deviation:

$\mu= \dfrac{\sin\lgroup\dfrac{\delta_m+A}{2}\rgroup}{\sin\lgroup\dfrac{A}{2}\rgroup}$

13.

Magnification Produced by Simple Microscope

$m= \lgroup1+\dfrac{D}{f}\rgroup$, where D = Normal viewing distance, f = focal length of the magnifying glass

14.

Magnification Produced by Compound Microscope

$m= -\dfrac{v_o}{u_o}\lgroup1+\dfrac{d}{f_o}\rgroup$

15. 

Magnification Produced by Telescope

$m = -\dfrac{f_o}{f_e}$

16.

Resolving Power of Optical Instruments

RP = $\dfrac{1}{\text{d}\theta} = \dfrac{D}{1.22\lambda}$