Reflection of Light and Mirrors
Reflection of Light and Mirrors
Light is the largest physics topic in the class 6 to 8 syllabus and reflection alone gives 2 questions. Mirror types with their uses, and the laws of reflection, are the two areas that repeat, and both can be tested unambiguously in a multiple-choice format.
The Laws of Reflection
- The angle of incidence equals the angle of reflection.
- The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
Angles are always measured from the normal, which is the perpendicular to the surface, never from the surface itself. Measuring from the surface is a standard error and gives the complement of the correct answer.
Regular reflection happens on smooth surfaces like a mirror and gives a clear image. Diffuse or irregular reflection happens on rough surfaces and is the reason we can see non-luminous objects from any direction. Both obey the same two laws — a point students find surprising.
✗ Diffuse reflection does not follow the laws of reflection | ✓ It follows them at every point; the surface is simply uneven
The Plane Mirror Image
| Property | Plane mirror image |
|---|---|
| Nature | Virtual and erect |
| Size | Same size as the object |
| Distance | As far behind as the object is in front |
| Sideways | Laterally inverted |
Lateral inversion is why the word AMBULANCE is painted reversed on the front of the vehicle, so a driver reads it correctly in the rear-view mirror. It is the standard example and is asked directly.
Two plane mirrors at an angle produce multiple images, and the number is given by 360 divided by the angle, minus one. At 60 degrees you get five images.
Spherical Mirrors
| Mirror | Reflecting surface | Image | Used in |
|---|---|---|---|
| Concave | Curves inward | Real or virtual, size varies | Headlights, shaving mirrors, dentist's mirror, solar cookers |
| Convex | Curves outward | Always virtual, erect, diminished | Rear-view mirrors, security mirrors |
The rule to remember is that a convex mirror always gives a small erect virtual image whatever the object distance, which is why it can show a wide field of view behind a vehicle. A concave mirror changes behaviour with distance: an object very close gives a magnified erect image, which is why it works as a shaving mirror.
Focal length = radius of curvature ÷ 2
Everyday Applications
- A periscope uses two plane mirrors at 45 degrees, and is used in submarines.
- A kaleidoscope uses three plane mirrors forming a triangular tube.
- Concave mirrors concentrate light and heat, which is how a solar cooker works.
- A dentist uses a concave mirror to see an enlarged image of a tooth.
These four devices are asked by name in a matching format, so learn which mirror each one uses rather than only what it does.
उत्तल दर्पण हमेशा छोटा, सीधा और आभासी प्रतिबिम्ब बनाता है।
TRE pointer: The mirror-and-use matching question is close to guaranteed, so learn the four applications above as pairs. The AMBULANCE lateral inversion example is the most quoted single item in the chapter. Note also that angles are measured from the normal, since a question giving the angle with the surface is a deliberate trap and the complement will be sitting among the options. With −1/3 negative marking, draw the ray before answering.
60-Second Recap
- Angle of incidence equals angle of reflection, both measured from the normal.
- Diffuse reflection still obeys the laws; the surface is merely uneven.
- A plane mirror gives a virtual, erect, same-size, laterally inverted image.
- Number of images between two mirrors at angle A is 360/A minus one.
- A convex mirror always gives a small erect virtual image, used in rear-view mirrors.
- Concave mirrors are used in headlights, shaving mirrors, dentists' mirrors and solar cookers.
Frequently Asked Questions
Why is AMBULANCE written in reverse on the vehicle?
Because of lateral inversion in a plane mirror. A driver ahead sees the word through the rear-view mirror, which reverses it left to right, so the reversed writing appears the right way round and can be read instantly. It is the standard example of lateral inversion.
Which mirror is used in a vehicle's rear-view mirror and why?
A convex mirror. It always produces a small, erect, virtual image regardless of how far the object is, and it covers a much wider field of view than a plane mirror of the same size. The trade-off is that objects appear closer than they are.
Does diffuse reflection obey the laws of reflection?
Yes, at every single point on the surface. The reflected rays scatter in many directions only because the surface is uneven, so the normals at different points face different ways. The laws are never broken, which is a point students commonly misunderstand.
From where is the angle of incidence measured?
From the normal, which is the line perpendicular to the surface at the point where the ray strikes. It is never measured from the surface itself. Questions that give the angle with the surface expect you to subtract from ninety before applying the law.