CBSE Class 10 Science • Light • Notes and Mind Map
Light Reflection and Refraction Class 10 Notes and Mind Map
Image formation becomes easier to understand when you follow what happens to light rays. These Light Reflection and Refraction Class 10 notes connect ray paths with mirrors, lenses, image characteristics and formulas. Use the mind map to organise the relationships and recall the reasoning behind each result.
Begin with the direction of light, predict where rays meet or appear to originate, and then use the relevant formula. A ray diagram and a calculation describe the same optical arrangement in different ways.
Reflection and Refraction: Follow the Ray
Reflection
Reflection returns light into the medium from which it arrived. The incident ray, reflected ray and normal lie in the same plane, and the angle of incidence equals the angle of reflection.
Refraction
When light enters a medium with a different refractive index, its speed changes. At oblique incidence, the direction generally changes too. The frequency remains unchanged at the boundary, while wavelength changes with speed.
Use Optical Density Correctly
An optically denser medium has a higher refractive index and a lower light speed. This does not simply mean greater mass density.
Real and Virtual Images: Where Do the Rays Go?
A real image forms where rays actually converge. A screen placed there can receive the image. A virtual image forms where the backward extensions of rays meet; the rays do not actually pass through that apparent position.
| Image type | Ray behaviour | Familiar example |
|---|---|---|
| Real | Outgoing rays converge at the image. | A convex lens projecting an image onto a screen. |
| Virtual | Outgoing rays appear to come from the image position. | An image seen in a plane mirror. |
A virtual image can be seen because rays still enter the eye. “Virtual” describes the apparent origin of those rays, not an absence of light.
Spherical Mirrors: Geometry and Focus
The pole is the midpoint of a spherical mirror’s reflecting surface. The centre of curvature is the centre of its parent sphere. The principal axis passes through both points.
Concave Mirror
Parallel rays close to the principal axis converge near the principal focus after reflection. Moving the object changes how the reflected rays meet and therefore changes the image.
Convex Mirror
Reflected parallel rays diverge and appear to originate from a focus behind the mirror. A real object produces a virtual, erect, diminished image.
Radius and Focal Length
For a spherical mirror using paraxial rays, f = R/2.
The focus lies halfway between the pole and centre of curvature in this approximation.
Image Formation as the Object Moves
A concave mirror and a convex lens share a useful pattern: a sufficiently distant real object produces a real, inverted image. Bringing the object towards the focus moves the real image farther away and increases its size.
When the object lies inside the focal distance, the outgoing rays diverge. Their backward extensions locate an enlarged virtual image.
| Situation | Concave mirror | Convex lens in air |
|---|---|---|
| Object beyond the focal distance | Real image forms in front of the mirror. | Real image forms on the opposite side of the lens. |
| Object at the focus | Reflected rays emerge parallel. | Refracted rays emerge parallel. |
| Object inside the focal distance | Virtual image appears behind the mirror. | Virtual image appears on the object’s side. |
Standard Rays: Why They Are Useful
Every point on an object sends light in many directions. Standard rays are selected because their paths are easy to construct. They locate the image without requiring every ray to be drawn.
- Concave mirror: a parallel ray reflects through the focus.
- Spherical mirror: a ray directed along a radius strikes normally and retraces its path.
- Convex lens: a parallel ray refracts through the far focus.
- Concave lens: a parallel ray emerges as if from the near focus.
- Thin lens: a ray through the optical centre passes approximately undeviated.
Refraction Through a Glass Slab
In a rectangular glass slab surrounded by the same medium on both sides, the emergent ray is parallel to the incident ray. It is displaced sideways because the two refractions occur at separated, parallel surfaces.
Parallel Does Not Mean the Same Path
The emergent ray can be parallel to the incident ray while following a different line. This sideways separation is lateral displacement.
Refractive Index: Connect Speed with Bending
Absolute refractive index is n = c/s, where c is light’s speed in vacuum and s is its speed in the medium. It has no unit because it is a ratio of two speeds.
Snell’s law is n₁ sin i = n₂ sin r. Angles are measured from the normal. It connects the refractive indices of the media with the ray directions.
Interpret a Refractive Index of 1.5
n = c/s
s = c/n
s = 3 × 10⁸ / 1.5
s = 2 × 10⁸ m/s
Light travels at two-thirds of its vacuum speed in this medium. The index describes speed, not how much light the material absorbs.
Sign Convention: Give Distances a Direction
Signed distances allow formulas to distinguish sides of an optical device. Measure mirror distances from the pole and thin-lens distances from the optical centre.
Distances along the incident-light direction are positive; distances opposite to it are negative. Heights above the principal axis are positive and those below it are negative.
| Relationship | Formula | Interpretation |
|---|---|---|
| Mirror formula | 1/f = 1/v + 1/u | Connects object, image and focal distances. |
| Mirror magnification | m = hᵢ/hₒ = −v/u | Describes orientation and relative size. |
| Thin-lens formula | 1/f = 1/v − 1/u | Uses the same distance convention with a different relationship. |
| Lens magnification | m = hᵢ/hₒ = v/u | Has no additional minus sign. |
| Lens power | P = 1/f | Use focal length in metres to obtain dioptres. |
Magnification and Lens Power
Positive magnification indicates an erect image relative to the object; negative magnification indicates an inverted image. The magnitude compares sizes: greater than one means enlarged, less than one means diminished.
Lens power measures converging or diverging ability. A shorter focal-length magnitude corresponds to greater power magnitude. A converging lens has positive power and a diverging lens negative power.
Separate the Sign from the Size
Magnification −0.5 means inverted and half the object’s size. The negative sign does not mean that the image has a physically negative length.
How to Use the Light Reflection and Refraction Mind Map
Use the map to connect the physical process, ray diagram and mathematical relationship. For each branch, explain where the rays go before recalling a formula.
- Reflection: recall the normal and angle relationships.
- Mirrors: connect curvature with convergence or divergence.
- Refraction: connect refractive index with speed and direction.
- Lenses: follow standard rays and identify image position.
- Formulas: recall the correct sign convention.
- Magnification and power: explain what the calculated values mean.
Redraw a small ray diagram from memory, then compare it with the notes. If the diagram and calculation disagree, check object position, distance signs and the chosen formula.
Frequently Asked Questions
What happens if half of a convex lens is covered?
The remaining portion can still form the complete image, but less light reaches it, so it becomes dimmer. Each lens portion receives rays from multiple object points.
Why can a virtual image be seen?
Light rays enter the eye and appear to originate from the virtual-image position. They need not actually converge at that position.
Can reflection produce both real and virtual images?
Yes. A concave mirror can produce either, depending on object position. A plane mirror gives a familiar virtual-image example.
Does a convex lens always form a real image?
No. A real object inside its focal distance produces a virtual, erect, enlarged image.
Why does a ray directed towards a mirror’s centre of curvature retrace its path?
It strikes along the radius, which is normal to the spherical surface. Its angle of incidence is zero.
Why is there no bending at normal incidence?
The incident and refracted angles are both zero. Speed and wavelength can still change at the boundary.
Is the slope of every current-style graph analogy enough to remember optical formulas?
Optical formulas should be learned with their own geometry and sign convention. Use ray diagrams to understand the relationships rather than unrelated graph analogies.
Why is a convex mirror useful for viewing a wider area?
Its diverging reflected rays allow a wide field of view. The resulting image of a real object is erect and diminished.
Does refractive index have a unit?
No. It is a ratio of quantities with the same units, so the units cancel.
Why must focal length be converted to metres for power?
The dioptre is defined as one reciprocal metre. Using centimetres directly gives an incorrect numerical value in dioptres.
What should I remember from each mind-map branch?
Recall the principle, one labelled diagram and the relevant formula. Explain how all three describe the same arrangement.
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