Stand still, face forward, and try to tell what is happening right behind you. You can’t, and the reason has nothing to do with your eyesight. It comes from the way light travels.
That simple fact is the doorway to the physics of reflection. A mirror does not give you new eyes. It redirects light so that your front-facing eyes can receive it. This guide follows that idea from the eye to the mirror, adds the laws of reflection and the properties of a plane mirror with two worked problems, and shows where reflection returns from Class 7 to Class 12.
No Light In, No Sight
Straight lines and a forward-facing eye
In a uniform medium such as air, light travels in straight lines, which is called the rectilinear propagation of light. Your eyes sit on the front of your head and face forward, so they can receive light only from the region ahead of you, roughly 180 to 200 degrees when both eyes are counted together.
What happens to light from behind you
Every object behind you scatters light in all directions. Some of it hits the back of your head and is blocked. Some travels forward past you, but it is moving away from your face. None of it can curve around your head and turn back into your pupils, so no light from directly behind you ever reaches your eyes.
Seeing is receiving
Most things around you are not sources of light. You see them because they scatter or reflect light from a lamp or the Sun, and some of that light enters your eyes. The eye is a receiver: it collects light and forms an image on the retina. It does not send anything out to “look” at things.
Memory anchor
Think “no light in, no sight.” If light from an object does not enter your eye, you cannot see that object, however close it is.
A Mirror Is a Rule About Angles
The two laws
When light strikes a smooth, polished surface, it bounces back into the same medium. This is reflection of light, and it obeys two laws.
- First law: the angle of incidence equals the angle of reflection, ∠i = ∠r.
- Second law: the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Both angles are measured from the normal, the line perpendicular to the surface at the point of incidence, and not from the surface itself.
Smooth or rough?
| Feature | Specular reflection | Diffuse reflection |
| Surface | Smooth and polished, like a mirror or still water | Rough, like paper, cloth or a wall |
| Reflected rays | Stay parallel to each other | Scatter in many directions |
| Laws of reflection | Obeyed at every point | Obeyed at every point |
| What you see | A clear image | The object itself, but no image |
Diffuse reflection is why you can see a wall from any angle, and specular reflection is why a mirror shows an image.
Seeing behind you, step by step
- An object behind you scatters light in all directions.
- Some rays travel forward and strike the mirror in front of you.
- The mirror reflects each ray according to the laws of reflection and sends it back towards you.
- Some of the reflected rays enter your eyes.
- Your brain assumes light travels in straight lines, so it traces the rays backwards and places the object behind the mirror.
Why the image sits behind the glass
No light actually exists behind the mirror. The reflected rays only appear to come from a point there, which makes the image a virtual image. Unlike a real image, it cannot be caught on a screen.
What a Plane Mirror Does to You
Four properties of the image
- It is virtual and erect.
- It is the same size as the object, so the magnification is +1.
- Its distance behind the mirror equals the object’s distance in front.
- It is laterally inverted.
Left-right or front-back?
A plane mirror reverses the image front to back, along the line perpendicular to its surface. Because we compare left and right by imagining ourselves turning around, this appears as a left-right swap, called lateral inversion. Up and down are not swapped.
The 2θ rule
If a plane mirror turns through an angle θ while the incident ray stays fixed, the reflected ray turns through 2θ. For example, when the mirror turns by 15°, the reflected ray turns by 30°. This single fact solves a whole family of numericals.
Worked example: the half-height mirror
What is the shortest plane mirror in which a person of height H, with eyes at height E above the floor, can see their full image?
- Feet: the ray from the feet must reflect into the eyes, so the mirror’s lower edge must be halfway between feet and eyes, at height E/2.
- Head: the upper edge must be halfway between the eyes and the top of the head, at E + (H − E)/2.
- Length: [E + (H − E)/2] − E/2 = H/2.
A mirror half your height is enough, and the answer does not depend on how far you stand from it.
Turning mirrors into tools
A periscope uses two plane mirrors, each tilted at 45°, to carry light around an obstacle so that you can see over it. For curved mirrors, read about concave and convex mirrors.
Reflection From Class 7 to Class 12
Reflection follows you through almost every year of school science. Class 7 introduces light, shadows and the image in a plane mirror (Light: Shadows and Reflections). Class 8 adds the laws of reflection and compares plane, concave and convex mirrors (Light: Mirrors and Lenses, Chapter 10). Class 10 adds image formation by spherical mirrors, the mirror formula and magnification (Light – Reflection and Refraction, Chapter 9 in the CBSE book and Chapter 10 in the Karnataka SSLC textbook). Class 12 reopens it all in Ray Optics and Optical Instruments.
Reflection of light, spherical mirrors and the mirror formula are listed in the NEET UG 2026 and JEE Main optics syllabi, and ray optics is part of the KCET syllabus. See the latest KCET physics syllabus and this guide to optics for NEET, and confirm sub-topics with your board’s current syllabus.
Frequently Asked Questions
Q1. Why can’t we see behind us without a mirror?
Light travels in straight lines, and our eyes receive only the light that enters them from the front. Light from objects behind us cannot bend around our head to reach our eyes.
Q2. How does a mirror let us see what is behind us?
It reflects light from objects behind us back towards our eyes, following the laws of reflection. Our brain traces those rays straight back and places the object behind the mirror, where a virtual image appears.
Q3. What are the laws of reflection?
The angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Q4. Why does my image look left-right reversed in a mirror?
A plane mirror reverses the image front to back, along the line perpendicular to its surface. We read that as a left-right swap, which is called lateral inversion.
Q5. Why is the image in a plane mirror as far behind the mirror as you are in front?
It follows from the equal angles in the laws of reflection. The reflected rays appear to diverge from a point as far behind the mirror as the object is in front, so the image sits at the same distance on the opposite side.
A Mirror Fixes What Light Cannot Do Alone
Light goes straight and our eyes face forward. A mirror is simply a surface that changes where the light goes next, so that our eyes can catch it. Keep that one picture in mind, and the laws, the image properties and the formulas that follow in later classes all grow from it.
If your child enjoys asking “why” about everyday things like this, the dSAT is a good first conversation with us.
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