The chapter Ray Optics deals with the study of Laws of Reflection, Deviation, Concept of Mirrors, Focal Principals, etc. The chapter holds the weightage of around 4% in NEET. Ray Optics is also known as Geometrical Optics.
In NEET 2021 examination, you can expect 2-3 questions from this chapter. The questions could be direct sales this chapter has various definitions and formulas from where direct questions of 4-8 marks could be asked. Also Check NEET Physics Exam Pattern
Read the article to know more details about various sub topics of Ray Optics.
As light travels through a homogenous medium along the straight-line path, it is called a Ray.
Reflection can be termed as a phenomenon where a ray of light, on hitting a boundary, is reverted back to the same medium it originated from. The boundary can be anything ranging from a rigid surface or an interface between two media.
Before we delve into the discussing Law of Reflection, it is of utmost importance to understand the following angles:
It is an angle that the incidence ray forms with normal at the point of incidence as illustrated in the diagram below:
∠i = ∠r
Q : What will be the angle of reflection for a ray of light incident towards a plane mirror at an angle of 30° with the mirror surface ?
Ans : 60°
The path of a ray of light changes as it gets reflected. The angle formed between its direction after reflection and the direction before reflection is known as Deviation as seen in the figure below.
If we rotate a reflecting surface by an angle 0 (anticlockwise) and keep the incidence ray fixed then there is reflect ray rotation by 20 along the same sense, which is, anticlockwise.
The image of an object can be seen behind the mirror after the object has been kept in front of the plane mirror. The distance of the object from the mirror equals the distance of the image from the mirror.
Parts of a Sphere are called spherical mirrors. The other surface of in them acts as a reflecting surface when one of the surfaces is painted silver.
It is a kind of spherical mirror that appears bulging at the edges and depressed at the center when it is looked from the reflecting side.
When this type of spherical mirror is looked from the reflecting side, it appears to be bulging at the center and depressed at the edges.
Q : A concave mirror gives real, inverted and same size image if the object is placed..
Ans (c) At c
What is Radius of Curvature?
It denotes that the focal length of a spherical mirror amounts to half of its radius of curvature.
Q : Image formed by plane mirror is
Ans : (c) Virtual and erect
(a) If the object is at infinity, u = ∞
v = -f , m = 0
Image obtained at the focus of a focal plane and small in dimensions.
(b) If object lies beyond centre of curvature,
2f > v >f
(c) If object is at centre of curvature,
v = -2f , m =1
Thus, a real and inverted image of same size as that of object gets formed at centre of curvature
(d) Object is in between a distance f and 2f , i.e., in between focus and centre of curvature (f < u < 2f):-
v < ∞ and m = (v/u) >1
Thus, a real, inverted and magnified image is made in between centre of curvature and infinity.
(e) Object is kept at focus (u = -f):-
v = -∞
Rendering of rays after reflection into a parallel beam in infinity
(f) Object is kept within focus (u < -f):-
v is positive.
In this way, a virtual, erect and magnified image is made on the other side of mirror
Q : Focal length of plane mirror is
Ans : (a) at infinity
Must Read:
In the phenomenon of Refraction, a ray of light passing from one medium to the other changes its velocity.
It is the ray that approaches the interface.
Ray that goes into another medium
(a)Snell’s Law : The sine of the angle of incidence shares a constant ratio with the sine of the angle of refraction.
sin i/sin r = constant
(b)The plane in which the Incident Ray, the Refracted Ray and the normal to the interface lie, is perpendicular to the interface separating the two media.
μ2/v – μ1/u = μ2 - μ1/R
μ2/v – μ1/u = μ2 - μ1/R
μ2/v – μ1/u = μ2 - μ1/R
Convex surface producing a real image of a real object:-
μ2/u – μ1/v = μ2 - μ1/R
μ/v – 1/u = μ-1/R
μ/u – 1/v = μ-1/R
f2 = μ2R/ [μ2 - μ2]
f1 = -μ1R/ [μ2 – μ1]
f2/v + f1/u = 1
A portion of refracting material bound between two spherical surfaces is called a lens.
A lens is said to be converging if the width of the beam decreases after refraction through it.
A lens is said to be diverge if the width of the beam increases after refraction through it.
Center of curvature of a surface of a lens is defined as the center of that sphere of which that surface forms a part.
Radius of curvature of a surface of a lens is defined as the radius of that sphere of which the surface forms a part.
1/f = 1/v – 1/u
It is the ratio between the size of the image to the size of the object.
m = I/O
Expression for m in terms of u, v and f :-
At f, u = -f. So, v = ∞
At C, u = 0, So, v = 0
1/f = (μ – 1) (1/R1 – 1/R2)
1/f = [[μ3 – μ1]/μ3R1] + [[μ3 – μ2]/μ3R2]
1/f = (μ – 1) (1/R1 – 1/R2)
1/f = [[μ3 – μ1]/μ3R1] + [[μ3 – μ2]/μ3R2]
F = f1f2 / f1+ f2
The reciprocal of the focal length of a lens, expressed in meter, is called its power.
P = 1/f
μ = sin [(A+dm)/2] / sin [A/2]
Here, dm is the minimum angle of deviation.
d = A (μ-1)
This signifies that the angle of deviation d is independent of the angle of incidence, provided it is small.
Q : The lens which diverges light from a single point is
Ans : (a) concave lens
Quick Links:
⦁ Dispersion:- The splitting of light into its constituent colors is called dispersion.
⦁ Cauchy’s formula:- μ = A+(B/λ2)+…
Here A and B are constants and λ is the wavelength of light.
⦁ Refraction through a prism:-
(a) Deviation:- A ray of monochromatic light (light possessing one wave-length only), while passing through a prism suffers a change in its path, the phenomenon is known as deviation.
d = (μ-1) A
Here A is the refractive angle of prism and μ is the refractive index of the material of prism for that particular wave length of light.
(b) Dispersion:- A ray of light (containing more than one wavelengths), while passing through the prism splits up into a number of rays. The phenomenon is called dispersion.
dv = (μv-1) A
dr = (μr-1) A
Here dv is deviation for violet and dr is the deviation for red color. μv and μr be the refractive indices of the material of prism for violet and red colors.
Since, μv > μr, therefore dv is greater than dr.
⦁ Dispersive power (ω):- Dispersive power of a prism is defined as the ratio between angular dispersion to mean deviation produced by the prism.
ω = (dv - dr)/d = (μv – μr)/(μ-1) = dμ/(μ-1)
⦁ Spectrum:- The band of colors lying side-by-side is called spectrum.
(a) Impure spectrum:- Impure spectrum is a spectrum in which the constituent colors overlap each other.
(b) Pure spectrum:- Pure spectrum is a spectrum in which all the constituent colors occupy different and distinct positions.
Sample Question
Q : With a prism (μ=1.5) having a refracting angle of 300, what will be the deviation of a monochromatic ray incident normally in its one surface will be ?
Ans: (a) 180 36’
⦁ Power of a concave lens (P):-
P = (100/x) dioptre, Here ‘x’ is the distance of far point of the defective eye, in ‘cm’.
⦁ Magnifying power or magnification of a simple microscope:-
? M = 1+ (D/f),
Here, ‘D’ is the distance of distinct vision and ‘f’ is the focal length.
⦁ Magnifying power or magnification of a compound microscope:-
? M = L/f0[1+(D/fe)]
f0 is the focal length of object, fe is the focal lengthy of eyepeice and L is the length of microscope tube.
⦁ Magnifying power or magnification of astronomical telescope (Normal Adjustment):-
M = f0/fe
Q : With an astronomical telescope having magnifying power for normal adjustment being 10 and the length being 110 cm, what would be the magnifying power of the telescope when the image is formed at the least distance of distinct vision for normal eye?
Ans : -14
⦁ Magnifying power or magnification of astronomical telescope (When the final image is formed at the distance of distinct vision):-
M = (f0/fe) [(fe+D)/D]
⦁ Magnifying power or magnification of Galileo’s telescope:-
M = F/f
Q : With an objective of focal length 100 cm and magnifying power 50, what will be the difference between the two lenses in normal adjustment in a Galileo telescope ?
Ans : 98 cm.
Q : What will be the angle of reflection if the angle of incidence is 30°?
Ans : 30°
Q : If the Convex lens focus on a real, point sized image then the object is placed
Ans : (c) at Infinity
Q :The radius of curvature of a mirror is 20cm the focal length is
Ans : (b) 10 cm
Q : With an object placed 10 cm from a concave mirror and focal length being 5 cm. what would be the image distance ?
Ans : 10 cm
Q : A lens which converges the incident rays on a single point is
Ans : (a) convex lens
Q : When white light is passed through a hollow prism then there is
Answer: (a) no dispersion and no deviation
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