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Light - How we see objects and the function of the eye

Grade 5IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Light sources and reflection: Luminous objects (like the Sun) emit light, while non-luminous objects are seen because light reflects off them into our eyes. The path of light can be represented as: Source→Object→Eye\text{Source} \rightarrow \text{Object} \rightarrow \text{Eye}.

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Light travels in straight lines: Light rays do not bend around corners. This is why shadows are formed when an opaque object blocks the light path, creating a region of 0 lx0\text{ lx} (lux) illumination.

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The Cornea and Lens: The cornea is the transparent front part of the eye that refracts light. The lens then fine-tunes this focus to ensure the image falls exactly on the retina.

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The Iris and Pupil: The iris is a muscle that controls the size of the pupil. In bright light, the pupil diameter dd decreases to protect the eye; in dim light, it increases to allow more light in.

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The Retina and Optic Nerve: The retina acts like a screen at the back of the eye containing light-sensitive cells. It converts light into electrical signals which are sent to the brain via the Optic Nerve\text{Optic Nerve}.

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Law of Reflection: When light hits a smooth surface (like a mirror), the angle at which it hits the surface is equal to the angle at which it bounces off: θi=θr\theta_i = \theta_r.

📐Formulae

Angle of Incidence (θi)=Angle of Reflection (θr)\text{Angle of Incidence } (\theta_i) = \text{Angle of Reflection } (\theta_r)

v=c≈3×108 m/sv = c \approx 3 \times 10^8 \text{ m/s}

Illuminance (lux)=Luminous flux (lumens)Area (m2)\text{Illuminance (lux)} = \frac{\text{Luminous flux (lumens)}}{\text{Area (m}^2)}

💡Examples

Problem 1:

A student is looking at a book in a dark room with a single lamp. Describe the path the light takes so the student can read, and identify the angle if the light hits the page at 30∘30^{\circ} to the normal.

Solution:

The light travels from the Lamp→Book→Eye\text{Lamp} \rightarrow \text{Book} \rightarrow \text{Eye}. If the angle of incidence θi=30∘\theta_i = 30^{\circ}, the angle of reflection θr=30∘\theta_r = 30^{\circ}.

Explanation:

Light must reflect off the non-luminous object (the book) to reach the eye. According to the Law of Reflection, θi\theta_i must equal θr\theta_r.

Problem 2:

Explain what happens to the pupil when a person walks from a dark cinema into bright sunlight.

Solution:

The Iris muscles will contract to make the Pupil diameter dd smaller.

Explanation:

The eye regulates the amount of light entering to prevent damage to the retina. In bright light, the aperture (pupil) must decrease in size.

Problem 3:

Why can we not see around a wooden block to look at a candle flame?

Solution:

Light travels in straight lines (180∘180^{\circ} path).

Explanation:

Because the wood is opaque and light cannot curve around it, the light rays from the candle are blocked, creating a shadow zone.