Light-Reflection, Mirrors & Lenses
📚 Topic Overview: LIGHT – Reflection, Mirrors & Lenses
In this topic, learners understand how light bounces back from surfaces (reflection),
how different mirrors and lenses form images, and how devices like kaleidoscopes and
periscopes work. These ideas are essential for explaining everyday phenomena and for
designing simple teaching aids in the classroom.
1. Mirrors and Reflection – Basic Ideas
Concept / Theory
Reflection is the bouncing back of light from a surface. A smooth and shining surface
like a mirror reflects most of the light in an orderly way. A plane mirror
forms an image that is virtual (cannot be taken on a screen), upright and of the same size
as the object. The distance of the image behind the mirror is equal to the distance of the
object in front of the mirror.
Analogy / Examples
- Throwing a ball on a smooth floor – the ball comes back in a regular way, similar to light reflecting from a smooth mirror.
- చిన్న పిల్లలు బావిలోకి చూసి తమ ముఖం కనిపించడాన్ని “అద్దం లాంటిదే” అని అనుభవిస్తారు – water acts like a natural mirror when it is still.
Conversions/Formulas
- Angle of incidence = Angle of reflection.
- Distance of object from mirror = Distance of image from mirror (for a plane mirror).
| English | Telugu |
|---|---|
| Reflection | ప్రతిఫలనం |
| Plane mirror | సమతల అద్దం |
| Virtual image | ఆభాస బింబం |
| Incident ray | పతన కిరణం |
| Reflected ray | ప్రతిఫలిత కిరణం |
In a barber shop, a plane mirror is used so that the person can see his face clearly,
with the same size and same upright position. The mirror does not enlarge or reduce
the face; it simply reflects it accurately.
When you see the words “same size, upright and cannot be taken on screen”,
the answer is usually plane mirror – virtual image.
Remember that a plane mirror never inverts the image upside-down; it only reverses left and right.
2. Regular and Diffused Reflection
Concept / Theory
When parallel rays of light fall on a smooth surface and reflect as parallel rays,
it is called regular reflection. This gives a clear image. When light
falls on a rough surface, the reflected rays scatter in many directions; this is called
diffused (irregular) reflection. Diffused reflection does not form a
clear image, but it helps in spreading light in a room.
Analogy / Examples
- Light from a torch on a new steel plate gives a clear bright patch – regular reflection.
- ఇటుక గోడ మీద వెలుగు పడితే అన్ని వైపులా చిందరవందరగా విస్తరిస్తుంది – this is diffused reflection on a rough wall.
Conversions/Formulas
- Surface type → Reflection type:
- Smooth & polished → Regular reflection
- Rough & uneven → Diffused reflection
- In both cases, each ray still obeys angle of incidence = angle of reflection,
but the directions differ due to the roughness.
| English | Telugu |
|---|---|
| Regular reflection | సాధారణ ప్రతిఫలనం |
| Diffused reflection | చెల్లాచెదురైన ప్రతిఫలనం |
| Smooth surface | మృదువైన ఉపరితలం |
| Rough surface | కొరతల ఉపరితలం |
| Scattering | చిందరవందర |
In a classroom, even if there is no mirror, the whole room becomes bright when
sunlight falls on the painted walls. The rough wall surface scatters light in all
directions (diffused reflection), so students sitting in different corners can see clearly.
If a question mentions “clear image” or “sharp reflection”, think of
regular reflection. If it mentions “no image, only brightness everywhere”,
then it is diffused reflection.
3. Multiple Images and Kaleidoscope
Concept / Theory
When two or more mirrors are placed at an angle, multiple images of an object are formed.
A kaleidoscope is an optical toy that uses three mirrors inside a tube at
certain angles, with coloured glass pieces, to create beautiful patterns. The number of
images depends on the angle between the mirrors.
Analogy / Examples
- Standing between two dressing mirrors in a cloth shop, a person sees many images of himself.
- రంగురంగుల గాజు ముక్కలతో పిల్లలు ఆడుకునే కలైడోస్కోప్ లో ఎన్నో అందమైన నమూనాలు కనిపిస్తాయి – these patterns are due to multiple reflections.
Conversions/Formulas
- Number of images (n) formed by two mirrors kept at angle θ (in degrees) can be found using:
n = (360° / θ) – 1 (for some simple cases in school level). - For θ = 60°, n ≈ (360/60) – 1 = 6 – 1 = 5 images.
| English | Telugu |
|---|---|
| Multiple images | అనేక బింబాలు |
| Kaleidoscope | కలైడోస్కోప్ (నమూనా బొమ్మ) |
| Angle between mirrors | అద్దాల మధ్య కోణం |
| Pattern | నమూనా |
| Multiple reflection | బహుళ ప్రతిఫలనం |
In a jewellery shop, mirrors are arranged on different walls. Because of multiple reflections,
the jewellery appears again and again, making the shop look fuller and more attractive
to customers.
Whenever two mirrors and “many images” are mentioned, think of
multiple reflections. Remember the approximate relation
n = (360/θ) – 1 for easy angles like 60°, 90°, 120° if needed.
4. Periscope – Use of Mirrors
Concept / Theory
A periscope is an instrument that uses two plane mirrors placed at 45° angles
inside a tube. It allows a person to see objects that are not in the direct line of sight,
such as above a wall or from inside a submarine. Light from the object reflects from the
top mirror to the bottom mirror, and then to the eye.
Analogy / Examples
- A person behind a wall using a periscope can see over the wall without being seen directly.
- రక్షణ దళాలు మరియు నావికాదళంలో నీటి అడుగున నడిచే నౌకలలో పై భాగం చూడడానికి పెరిస్కోప్ ఉపయోగిస్తారు – it is useful for safe observation.
Conversions/Formulas
- Periscope design → uses two plane mirrors at 45°.
- Path of light: Object → Top mirror → Bottom mirror → Eye (two reflections).
| English | Telugu |
|---|---|
| Periscope | పెరిస్కోప్ |
| Plane mirrors | సమతల అద్దాలు |
| Observation | పరిశీలన |
| Submarine | నీటి అడుగు నౌక |
| Line of sight | దృశ్య రేఖ |
In some school science exhibitions, students build simple periscopes using cardboard
and two small mirrors. With this, they can see over a classroom partition or behind
a crowd without standing up.
If a question says “seeing above an obstacle using mirrors in a tube”,
the answer is almost always periscope. Remember: two plane mirrors,
each at 45° to the tube.
5. Characteristics of Images Formed by Mirrors
Concept / Theory
Different mirrors form images with different characteristics. A plane mirror forms a
virtual, upright image of the same size, with left-right reversal (lateral inversion).
Concave and convex mirrors (spherical mirrors) can form images with different sizes and
positions depending on where the object is placed.
Analogy / Examples
- In a dressing mirror, your height and size look the same – this is characteristic of a plane mirror.
- చిన్న వంకర అద్దం (convex mirror) లో చూసినప్పుడు మన ముఖం చిన్నగా కానీ ఎక్కువ ప్రాంతం కనిపిస్తుంది – used on vehicles.
Conversions/Formulas
- Plane mirror: Image size = Object size; Distance of image = Distance of object.
- Lateral inversion: Left appears right, right appears left.
| English | Telugu |
|---|---|
| Lateral inversion | పార్శ్వ మార్పు |
| Upright (erect) | నిలువుగా ఉన్న |
| Image size | బింబ పరిమాణం |
| Image distance | బింబ దూరం |
| Object distance | వస్తు దూరం |
The word “AMBULANCE” is written in reverse on the front of the vehicle. In the driver’s
mirror of a car ahead, it appears correctly. This is a daily example of lateral inversion.
Whenever you read “left-right reversed but same size and upright”, your mind should
immediately connect to the plane mirror. This is a favourite question area.
6. Spherical Mirrors and Images
Concept / Theory
Spherical mirrors are parts of a sphere. The inside reflecting mirror is
called a concave mirror, and the outside reflecting mirror is a
convex mirror. Concave mirrors can form real as well as virtual images
depending on the object’s position. Convex mirrors always form virtual, upright and diminished
(smaller) images.
Analogy / Examples
- Concave mirror: The inner shining surface of a spoon acts like a concave mirror.
- Convex mirror: The outer shining surface of a steel spoon acts like a convex mirror – చిన్న బింబం కానీ ఎక్కువ ప్రాంతం కనిపిస్తుంది.
Conversions/Formulas
- Concave mirror: Used as shaving mirror, dentist mirror – can produce enlarged image when the object is close.
- Convex mirror: Used as rear-view mirror in vehicles – small image but large area view.
| English | Telugu |
|---|---|
| Concave mirror | అవతల అద్దం |
| Convex mirror | అయతల అద్దం |
| Real image | నిజ బింబం |
| Diminished image | చిన్నదైన బింబం |
| Enlarged image | పెద్ద బింబం |
At a hair-cutting salon, the mirror used to show the back of the head is often slightly
concave, so that the back appears a little larger and clearer. On vehicles, the side mirrors
are usually convex to show more area of the road.
A common question: “Which mirror is used as rear-view mirror?” Answer:
Convex mirror. Clue words – “wide field of view”, “small images of many vehicles”.
7. Lenses and Images
Concept / Theory
A lens is a transparent piece of glass or plastic with two curved surfaces.
A convex lens is thicker in the middle and converges light rays, while a
concave lens is thinner in the middle and diverges light rays. Convex lenses
can form real or virtual images; concave lenses generally form virtual, upright and diminished images.
Analogy / Examples
- Convex lens: A magnifying glass used to enlarge print – thicker at centre.
- కళ్ళజోడులో కొందరికి మధ్యలో మందంగా ఉండే గాజు (convex) – for long-sight correction; మధ్యలో పలుచగా ఉండే గాజు (concave) – for short-sight correction.
Conversions/Formulas
- Convex lens: Converging lens – can focus sunlight to a point.
- Concave lens: Diverging lens – spreads light rays out.
| English | Telugu |
|---|---|
| Lens | లెన్స్ / గాజు లెన్స్ |
| Convex lens | అవతల లెన్స్ |
| Concave lens | అయతల లెన్స్ |
| Converging | కేంద్రీకరించే |
| Diverging | చెల్లాచెదురుగా పంపించే |
Children sometimes use a convex lens to focus sunlight on a piece of paper and burn a small
hole. This shows that the lens brings many rays of the Sun to one point – a practical example
of a converging lens.
Keywords to remember: Convex → converging → can burn paper;
Concave → diverging → used in spectacles for short-sighted eyes.
These word links help in identifying correct options quickly.
8. Summary & Memory Formula Sheet (Reflection, Mirrors & Lenses)
Concept / Theory
Reflection, mirrors and lenses together explain many visual experiences in daily life,
from seeing ourselves in a mirror to using spectacles and vehicle mirrors. Understanding
simple rules, image properties and real-life uses helps in both content questions and
pedagogy-based questions in school examinations.
Analogy / Examples
- House design uses diffused reflection (wall colours) and plane mirrors (decorative mirrors) to manage light.
- కళ్ళజోడు, వాహన అద్దం, రంగుల బొమ్మల కలైడోస్కోప్ – all are everyday learning tools for children to understand light behaviour.
Conversions/Formulas
- Angle of incidence = Angle of reflection (for every reflected ray).
- Plane mirror: Image distance = Object distance; image size = object size.
- Two mirrors: Approximately, number of images n ≈ (360° / θ) – 1 (for simple angles).
- Convex lens: Converging; Concave lens: Diverging.
- Convex mirror: Small, upright image; Concave mirror: can give enlarged image when object is close.
| English | Telugu |
|---|---|
| Regular reflection | సాధారణ ప్రతిఫలనం |
| Diffused reflection | చెల్లాచెదురైన ప్రతిఫలనం |
| Periscope | పెరిస్కోప్ |
| Concave / Convex mirror | అవతల / అయతల అద్దం |
| Convex / Concave lens | అవతల / అయతల లెన్స్ |
A teacher can bring a spoon, a small mirror, a convex lens and a piece of frosted glass to the
classroom. By letting children look at their images and at the blackboard through each object,
the teacher can demonstrate mirrors, spherical mirrors, lenses and diffused reflection in a
simple, activity-based way.
While answering questions, first identify which device the question is about
(plane mirror, spherical mirror, lens, periscope, kaleidoscope). Then recall its fixed
key property – for example, convex mirror → small upright image + wide field of view;
convex lens → converging; plane mirror → same size, upright, lateral inversion. Matching device
to its key property helps eliminate wrong options quickly.