The core idea
Reflection returns light into its original medium. Refraction changes its propagation as it enters another medium with a different light speed. Measure ray angles from the normal and use actual ray paths to explain images.
1. Use a ray as a model of direction
A luminous object produces light; an ordinary book becomes visible because it reflects some incident light toward our eyes. Light is an electromagnetic wave and can travel through a vacuum. For many everyday optical situations, a ray is a useful line showing its travel direction. In a uniform transparent medium, rays follow straight paths. Arrows on a ray point from the light source toward whatever receives the light, not outward from the eye. A diagram normally selects a few useful rays from the many leaving an object point. Rays are a model; drawing only two does not mean that the object emits or reflects only two.
Sources: NCERT: Light—Reflection and Refraction ↗ · NCERT: Waves ↗
2. Measure reflection from the normal
At the point where a ray strikes a surface, draw a line perpendicular to that surface: the normal. The angle of incidence i lies between the incoming ray and the normal; the angle of reflection r lies between the reflected ray and the normal. Reflection obeys i = r, and the two rays and normal lie in one plane. A smooth mirror sends nearby parallel rays in an organised reflected direction. A rough surface has many differently tilted local normals, so it sends light in many directions. Each small region still obeys the reflection law. This diffuse reflection is why many people can see the same page from different positions.
3. Explain an image behind a plane mirror
Choose one point on an object and trace two rays that reflect from a plane mirror into an observer’s viewing region. Extend the reflected rays backward as dashed lines. The extensions meet behind the mirror, where the light appears to originate. Actual reflected rays do not meet there, so the image is virtual. A plane mirror gives an upright image of the same size, at the same perpendicular distance behind the mirror as the object is in front. Each object point has a corresponding image point. A screen behind the mirror cannot catch this image because the reflected light does not actually pass through those apparent points.
Real rays, apparent meeting point
4. Connect refraction with a change of speed
When light passes between transparent media, its speed can change. At oblique incidence, the transmitted ray generally changes direction too. Entering a higher-refractive-index medium, it bends toward the normal; entering a lower-index medium, a transmitted ray bends away. At normal incidence it can change speed without bending. Absolute refractive index is n = c/v, where c is vacuum light speed and v is speed in the medium. Both speeds use the same units, so n has no unit. “Optically denser” means a higher refractive index, not necessarily more mass per volume. Refraction explains why light from an underwater point can seem to come from a shallower apparent position.
5. Let curved surfaces redirect a bundle
A lens refracts light at its surfaces. For ordinary glass lenses in air, a convex lens can bring rays parallel to its principal axis together at a focus; a concave lens spreads them as if from a focus. Focal length is the distance from a thin lens’s optical centre to the principal focus. With a real object beyond the focal length, a converging lens can form a real, inverted image where rays actually meet. With the object closer than its focal length, it gives an upright virtual image. Thus “convex” does not always mean “real image.” Paper ray diagrams can explore these cases using parallel rays and rays approximately undeviated through the thin lens’s centre.
6. Worked example: mirror distances and angles
Illustrative model: a student stands 1.20 m perpendicularly in front of a plane mirror. The image is 1.20 m behind it, so student-to-image separation is 2.40 m. If the student moves 0.30 m directly toward the mirror, the new object distance is 0.90 m and separation becomes 1.80 m. The separation decreased by twice the student’s movement. In the same paper model, a ray meeting the mirror at 30° to the normal reflects at 30° to the normal. Each angle to the mirror surface is 60°, because normal and surface are perpendicular. Always label which line defines an angle.
7. Worked example: interpreting refractive index
For a hypothetical transparent sample and one specified colour, take v = 1.875 × 10⁸ m/s and approximate c = 3.00 × 10⁸ m/s. Then n = c/v = 3.00/1.875 = 1.60; the shared factor 10⁸ m/s cancels. The sample speed is c/1.60, or 62.5% of c. Light entering this sample obliquely from air, whose index we approximate as 1, bends toward the normal. This ratio alone does not give a bending angle without an incidence angle and the refraction law. At perpendicular entry, the speed still changes but the direction remains straight. The numerical sample is illustrative, not a claim about all glass.
PUT IT INTO PRACTICE
Apply it and check your reasoning
- Make a paper diagram of a plane mirror, its normal, and an incident ray at an illustrative 45° to the normal. Draw the reflected ray and label actual travel with solid arrows.
- Place an object point 0.80 m in front in your labelled model, mark the virtual image, and calculate their separation. Separately calculate v for a hypothetical n = 1.25 using c = 3.00 × 10⁸ m/s.
- Check: reflection angle is 45°, image distance is 0.80 m behind, separation is 1.60 m, and v = 2.40 × 10⁸ m/s. Use dashed extensions behind the mirror because those are apparent paths.
Check your understanding
Why can a book be seen from many directions?
Its rough surface reflects incident light into many directions. Different local normals produce different reflected directions while obeying the same law.
Is a virtual image imaginary or unobservable?
No. It can be seen because real rays reach the eye. “Virtual” describes where backward extensions meet, rather than an actual meeting of those rays.
Can light change speed without changing direction?
Yes. At normal incidence between suitable transparent media, it keeps a straight direction while its speed changes.
Does higher refractive index necessarily mean higher mass density?
No. Refractive index compares light speeds. Mass density is mass per volume; these are distinct material properties.
Why can one convex lens produce either a real or a virtual image?
Object distance changes whether emerging rays converge or instead diverge with intersecting backward extensions. The lens type alone does not settle image type.