Light Shadows and Reflections
Light lets us see, casts shadows and bounces off mirrors. This Class 7 Curiosity chapter shows that light travels in straight lines, explains how shadows form, sorts objects as transparent, translucent or opaque, and describes reflection and the image we see in a plane mirror.
Learning objectives
- Show that light travels in a straight line.
- Explain how shadows are formed.
- Classify objects by how light passes through them.
- Describe reflection and plane-mirror images.
Key concepts
Light travels in straight lines
Light travels in straight lines, a fact we can show with the way it streams through a small hole or a set of aligned holes. Because of this straight-line travel (rectilinear propagation), we cannot see around corners, and a pinhole can form an image. We see objects when light from them reaches our eyes.
Shadows
A shadow forms when an opaque object blocks the straight path of light, leaving a dark region behind it. A shadow shows the outline (shape) of the object but not its colour or detail. Its size and direction change as the position of the light source changes.
Transparent, translucent and opaque
Objects are grouped by how much light passes through them. Transparent objects, like clear glass, let almost all light through so we can see clearly. Translucent objects, like frosted glass, let some light through but we cannot see clearly. Opaque objects, like wood and metal, let no light through and so cast sharp shadows.
Reflection and images in a plane mirror
When light falls on a smooth, shiny surface like a mirror, it bounces back; this is reflection. A plane mirror forms an image that is upright, the same size as the object, and as far behind the mirror as the object is in front. The image also shows left–right reversal, called lateral inversion.
Key definitions
- Rectilinear propagation
- The travelling of light in straight lines.
- Opaque
- Not allowing light to pass through.
- Reflection
- The bouncing back of light from a surface.
- Lateral inversion
- The left–right reversal of an image in a plane mirror.
Solved examples
Q1. What is needed to form a shadow?
Solution: A source of light and an opaque object blocking it.
Q2. Is frosted glass transparent, translucent or opaque?
Solution: Translucent — it lets some light through but you cannot see clearly.
Q3. How far behind a plane mirror is the image?
Solution: As far behind as the object is in front.
Common mistakes to avoid
- Thinking a shadow shows the object's colour (it shows only the outline).
- Confusing translucent with transparent.
- Believing the mirror image is magnified (it is the same size).
- Forgetting that a plane-mirror image is laterally inverted.
Light Shadows and Reflections — MCQ Quiz
10 questions with instant feedback. Use number keys 1–4 to answer.
Light travels in:
Practice questions
Short answer
In what kind of path does light travel?
In straight lines (rectilinear propagation).
What is reflection?
The bouncing back of light from a surface.
What is lateral inversion?
The left–right reversal of an image in a plane mirror.
Long answer
Explain how shadows are formed and what they tell us about light.
A shadow is the dark region formed behind an opaque object when it blocks the path of light. Because light travels in straight lines, it cannot bend around the object, so the region directly behind it receives no light and appears dark. A shadow shows only the outline or shape of the object, not its colour or surface detail, which is why shadows of differently coloured objects of the same shape look alike. The size and direction of a shadow change with the position of the light source — a light source close to the object or low down produces a longer shadow. Shadows are thus direct evidence that light travels in straight lines.
Describe the image formed by a plane mirror.
When light falls on the smooth, shiny surface of a plane mirror it is reflected, and this reflected light forms an image. The image in a plane mirror has several definite features: it is upright (the same way up as the object), it is the same size as the object, and it appears to be as far behind the mirror as the object is in front of it. The image is also laterally inverted, meaning left and right are swapped — this is why the word written on the front of an ambulance appears reversed except when seen in a mirror. Such an image cannot be caught on a screen, so it is called a virtual image.
HOTS (Higher Order Thinking)
Why does your shadow become longer in the early morning and evening?
Because the Sun is low in the sky then, so light strikes at a slant and the opaque body blocks a longer region of light.
Why is the word 'AMBULANCE' often written reversed on the front of the vehicle?
So that drivers ahead see it the right way round in their rear-view mirror, because a plane mirror laterally inverts the image.
Quick revision
Revision notes
- Light travels in straight lines; we see when light reaches our eyes.
- Shadow = opaque object blocking light; shows outline only.
- Transparent (clear) / translucent (partly) / opaque (none).
- Plane mirror image: upright, same size, equal distance behind, laterally inverted.
Key takeaways
- Straight-line travel of light explains shadows.
- Objects are transparent, translucent or opaque.
- Plane-mirror images are same-size and laterally inverted.
Frequently asked questions
Why can't we see around corners?
Because light travels in straight lines and cannot bend around obstacles.
What is the difference between translucent and opaque?
Translucent lets some light through; opaque lets none through.
Is a mirror image bigger than the object?
No — in a plane mirror it is the same size.