Light – Reflection and Refraction – Very Short Answer Type Questions
CBSE Class 10 Physics — Chapter 9: Light — 60 Very Short Questions & Answers
Class: CBSE Class 10
Subject: Physics
Chapter: Chapter 9 — Light: Reflection & Refraction
CBSE Board Examinations
Systematic order: Syllabus → Key concepts → Topic-wise Q&A → Quick revision
Instructions: 60 very short answer questions with concise, NCERT-aligned answers for quick revision of Chapter 9 — Light: Reflection & Refraction. Use for last-minute revision and practice.
Ray Model & Basic Terms (Q1–10)
1
What is a light ray?
A line representing the direction of propagation of light.
2
Define beam of light.
A bundle of light rays; can be parallel, divergent or convergent.
3
What is an incident ray?
A ray striking a surface before reflection or refraction.
4
What is the normal at a point on a surface?
A line perpendicular to the surface at the point of incidence.
5
Define reflected ray.
Ray that bounces off a surface after reflection.
6
Define refracted ray.
Ray that bends and travels in the second medium after refraction.
7
What is the principal axis?
A line through pole and centre of curvature (for mirrors) or optical centre (for lenses).
8
What is the point called where reflected rays appear to meet for a plane mirror?
Virtual image point behind the mirror.
9
What is lateral inversion?
Left-right reversal of image in a plane mirror.
10
What does 'virtual image' mean?
An image from which rays appear to diverge but do not actually meet; cannot be formed on a screen.
Reflection Laws & Plane Mirror (Q11–20)
11
State the two laws of reflection.
(1) Incident ray, reflected ray and normal lie in same plane. (2) Angle of incidence = angle of reflection (i = r).
12
Image size in a plane mirror compared to object?
Same size as the object.
13
Distance of image from plane mirror relative to object?
Image distance equals object distance (v = u behind mirror).
14
Is the image from a plane mirror erect or inverted?
Erect (upright).
15
Can a plane mirror produce a real image?
No; plane mirrors produce virtual images only.
16
What is lateral inversion common example?
Left-right swap seen in plane mirror reflections of text/people.
17
Why are mirrors used for decoration — short reason?
They form erect images of same size, creating pleasant visual effects and illusion of space.
18
What is the pole of a spherical mirror?
The central point on the mirror surface (denoted P).
19
Define centre of curvature (C).
Centre of the sphere of which the mirror is a part; radius = R.
20
Define focal point (F) for a mirror.
Point where parallel rays converge (concave) or appear to diverge from (convex); f = R/2.
Spherical Mirrors & Formulae (Q21–30)
21
Give the mirror formula.
1/v + 1/u = 1/f (where u = object distance, v = image distance, f = focal length).
22
State magnification formula for mirrors.
m = hi/ho = v/u (negative m → inverted image).
23
Sign of focal length for concave mirror?
Focal length (f) is negative using Cartesian sign convention for concave mirror (depending on convention used; NCERT examples often take f negative for concave). Follow textbook convention.
24
If object at infinity, image formed at?
At focal point F (real image at f).
25
Where is image when object is between F and mirror (concave)?
Virtual, erect and magnified image behind the mirror.
26
What type of image when object beyond C (concave)?
Real, inverted and diminished image between F and C.
27
Define radius of curvature (R).
Radius of the sphere of which the mirror segment is part; R = 2f.
28
What image does a convex mirror always produce?
Virtual, erect and diminished image behind the mirror.
29
Why are convex mirrors used as vehicle rear-view mirrors?
They give a wide field of view and diminished virtual images so driver sees more area.
30
How to find focal length experimentally for a concave mirror using distant object?
Focus parallel rays from distant object to form image at focal point; measure distance PF ≈ f.
Refraction & Snell's Law (Q31–40)
31
What is refraction of light?
Bending of light when it passes from one medium to another due to speed change.
32
State Snell's law.
n1 sin i = n2 sin r, where n = refractive index, i = incidence angle, r = refraction angle.
33
How is refractive index n defined (speed form)?
n = c / v (c = speed of light in vacuum, v = speed in medium).
34
Which medium has higher refractive index: glass or air?
Glass has higher refractive index than air (light slower in glass).
35
What is apparent depth?
Depth at which an object appears due to refraction (object in water appears raised).
36
Formula relating real and apparent depth (approx.)?
n ≈ real depth / apparent depth for near-normal incidence (air–water interface).
37
What happens to a ray when it enters a denser medium from rarer medium?
It bends toward the normal.
38
What is refraction through a glass slab — net lateral shift or deviation?
Ray emerges parallel to incident but displaced laterally (lateral shift).
39
Does wavelength change on refraction?
Yes — wavelength decreases in denser medium; frequency remains same.
40
What stays constant when light passes into another medium?
Frequency of light remains unchanged; speed and wavelength change.
Total Internal Reflection & Optical Fibres (Q41–50)
41
What is total internal reflection (TIR)?
Complete reflection of light back into denser medium when incidence angle > critical angle.
42
Condition for TIR to occur?
Light must travel from denser to rarer medium and angle of incidence > critical angle θc.
43
Formula for critical angle?
sin θc = n2 / n1 (for n1 > n2).
44
Why are optical fibres useful?
They guide light by TIR, enabling low-loss transmission for communications and medical endoscopes.
45
Name two parts of an optical fibre.
Core (high n) and cladding (lower n).
46
What is numerical aperture (NA) in simple terms?
It measures light-gathering ability of fibre; related to acceptance angle.
47
Give one medical application of optical fibres.
Endoscopy — internal imaging using fibre bundles.
48
What happens if cladding is damaged?
Light leaks out; transmission efficiency drops and signal lost.
49
Can TIR occur from air to glass?
No; TIR requires light going from denser (glass) to rarer (air), not vice versa.
50
State one advantage of fibre optics over copper wires.
Higher bandwidth and lower signal loss over long distances.
Lenses, Ray Diagrams & Practical Tips (Q51–60)
51
What are convex and concave lenses in one line?
Convex (converging) lenses bulge out and can form real images; concave (diverging) lenses curve in and form virtual images.
52
State the thin lens formula.
1/v − 1/u = 1/f (v = image distance, u = object distance, f = focal length).
53
How is magnification for a lens given?
m = hi/ho = v/u (negative m indicates inverted image).
54
What is power of a lens and its unit?
P = 1/f (f in metres); unit = dioptre (D).
55
Which ray from an object through centre of a thin lens does what?
Ray through optical centre passes undeviated (approximately).
56
How to get a real image with a convex lens?
Place object beyond focal length; real inverted image forms on other side.
57
What is virtual image with a lens?
When object is within focal length of convex lens, image formed is virtual, erect and magnified.
58
Give one practical: how to find focal length of a convex lens using sunlight.
Focus distant sunlight on a screen; distance from lens to focused spot ≈ focal length.
59
Name two common ray rules used for drawing lens diagrams.
Parallel ray → passes through focus; ray through centre → undeviated.
60
List one important exam tip for ray diagrams.
Draw principal axis, label O, I, F, C and show at least two rays with arrows for clarity.