What is the difference between how a red filter and a red opaque object produce colour?

A red filter produces colour by **transmitting** red light and absorbing other wavelengths, whereas a red opaque object produces colour by **reflecting** red light and absorbing others.

How does the interaction of light with a white object differ from that with a black object?

A white object **reflects all** visible wavelengths equally, whereas a black object **absorbs all** visible wavelengths and reflects none.

Compare the refraction of violet light and red light when passing through a prism.

Violet light has a shorter wavelength and is refracted (bent) **the most**, while red light has a longer wavelength and is refracted **the least**.

What common error occurs when predicting the colour of a blue object under red light?

Students often incorrectly guess the object will appear purple or dark blue. In reality, the object appears **black** because it absorbs the red light and there is no blue light present to reflect.

What misconception do students often have about what colour filters do to white light?

A common misconception is that filters 'dye' or add colour to the light. In reality, filters work by **subtracting** (absorbing) all colours except the one they transmit.

Why is it incorrect to say a black object 'emits' black light?

Black is not a colour of light but the **absence** of light. A black object appears black because it absorbs all incident light and reflects nothing to the eye.

What is the definition of a transparent object in terms of light interaction?

A transparent object is a material that **transmits** all (or nearly all) incident light, with negligible reflection or absorption, allowing objects behind it to be clearly seen.

What is the visible light spectrum?

The visible light spectrum is the range of electromagnetic wavelengths that the human eye can detect, perceived as different colours ranging from red (longest wavelength) to violet (shortest).

Define 'opaque' in the context of light.

An opaque object is a material that does not allow light to pass through it (transmit); instead, it either **reflects** or **absorbs** the incident light.

What determines the specific colour of an opaque object?

The colour is determined by the specific wavelengths of light that are **strongly reflected** by the surface; all other wavelengths are absorbed.

Library Podcasts

Courses

Referral & Rewards

Colour & Reflection of Light

Colour Perception and Light Interaction

Summary

This topic explores how the visible colour of objects and light sources is determined by the interaction of specific wavelengths with matter. It covers the composition of white light, the subtractive nature of colour filters, and the selective reflection that defines the colour of opaque surfaces.

1. Composition of White Light

White light is not a single colour but a composite mixture of all visible wavelengths in the electromagnetic spectrum.

Dispersion occurs when white light passes through a prism, separating into constituent colours due to refraction.

Different colours refract by different amounts based on their wavelength: Violet light (shortest wavelength) refracts the most, while Red light (longest wavelength) refracts the least.

This separation principle confirms that spectral colours exist simultaneously within white light.

2. Transmission Through Colour Filters

Filters operate via a subtractive process: they absorb specific wavelengths and transmit others.

The colour of a filter is defined by the wavelength of light it transmits.

For example, a blue filter transmits blue wavelengths and absorbs all other wavelengths (red, green, yellow, etc.).

If white light shines on a filter, the emerging light consists only of the transmitted wavelengths.

Diagram showing white light components hitting a green surface. The green wavelength reflects off, while red and blue wavelengths stop at the surface, indicating absorption.

3. Reflection from Opaque Surfaces

The colour of an opaque object is determined by the wavelengths of light it reflects into the observer's eye.

Selective Reflection: An object appears a specific colour because it reflects that specific wavelength and absorbs all others.

Conservation of Light: Incident Light = Reflected Light + Absorbed Light (assuming no transmission).

A red apple appears red because it reflects red wavelengths and absorbs blue, green, and yellow wavelengths.

4. Key Distinctions: White, Black, and Transparent

5. Exam Strategy & Tips

Colour Perception and Light Interaction

Summary

1. Composition of White Light

White light is not a single colour but a composite mixture of all visible wavelengths in the electromagnetic spectrum.

Dispersion occurs when white light passes through a prism, separating into constituent colours due to refraction.

Different colours refract by different amounts based on their wavelength: Violet light (shortest wavelength) refracts the most, while Red light (longest wavelength) refracts the least.

This separation principle confirms that spectral colours exist simultaneously within white light.

2. Transmission Through Colour Filters

Filters operate via a subtractive process: they absorb specific wavelengths and transmit others.

The colour of a filter is defined by the wavelength of light it transmits.

For example, a blue filter transmits blue wavelengths and absorbs all other wavelengths (red, green, yellow, etc.).

If white light shines on a filter, the emerging light consists only of the transmitted wavelengths.

Diagram showing white light components hitting a green surface. The green wavelength reflects off, while red and blue wavelengths stop at the surface, indicating absorption.

3. Reflection from Opaque Surfaces

The colour of an opaque object is determined by the wavelengths of light it reflects into the observer's eye.

Selective Reflection: An object appears a specific colour because it reflects that specific wavelength and absorbs all others.

Conservation of Light: Incident Light = Reflected Light + Absorbed Light (assuming no transmission).

A red apple appears red because it reflects red wavelengths and absorbs blue, green, and yellow wavelengths.

4. Key Distinctions: White, Black, and Transparent

White Objects: Reflect all visible wavelengths equally. No specific colour is absorbed.

Black Objects: Absorb all visible wavelengths. No light is reflected to the eye, which the brain interprets as black (the absence of light).

Transparent Objects: Transmit all incident light. Very little light is reflected or absorbed, allowing objects behind them to be seen clearly.

5. Exam Strategy & Tips

Trace the Wavelengths: When solving problems involving coloured lights and objects, explicitly list which wavelengths are present in the source, which are transmitted by any filter, and which are reflected by the object.

The 'Black' Trap: If an object is illuminated by a colour of light it normally absorbs (e.g., a red ball in blue light), it will appear black, not purple or dark red. The object absorbs the blue light, and since there is no red light to reflect, no light reaches the eye.

Filter vs. Surface: Remember that filters transmit (pass through) light, while opaque surfaces reflect (bounce back) light. Do not confuse the two mechanisms.

Sanity Check: You cannot reflect a colour that wasn't in the incident light. A blue object cannot look blue if lit by pure red light.