In basic physics of light and colour, a blackboard appears black in colour because of the way it absorbs and reflects visible light falling on its surface.

Introduction / Context:
Colour perception in daily life is explained by the interaction between light and objects. A blackboard is a familiar classroom object that appears black, and understanding why it looks black helps clarify how absorption and reflection of light work. This question examines the basic idea that black objects absorb nearly all colours of visible light rather than reflecting them.

Given Data / Assumptions:

• White light from a tube light or the Sun consists of many colours.
• Objects appear coloured based on which wavelengths of light they reflect and which they absorb.
• A blackboard is coated with a dark material designed to give a black appearance.
• The human eye sees an object because of the light reflected from it.

Concept / Approach:
When white light falls on an object, some wavelengths may be absorbed and others reflected. If an object reflects a particular colour strongly and absorbs other colours, the eye perceives that object as having the reflected colour. For example, a red surface reflects red light more and absorbs other colours. A black surface, however, absorbs most of the visible light that falls on it and reflects very little. Because almost no light is reflected back to the eye, the surface appears black. Therefore a blackboard appears black because it is a strong absorber of visible light, not because it reflects black light, which is not a separate colour in the visible spectrum.

Step-by-Step Solution:
Step 1: Recall that black colour indicates an absence of reflected visible light reaching the eye. Step 2: Understand that a blackboard surface is designed to absorb most of the visible light incident on it. Step 3: Note that if an object reflected all colours of visible light equally, it would look white, not black. Step 4: Recognise that the idea of reflecting black colour is misleading because black corresponds to almost no reflected light. Step 5: Conclude that the correct explanation is that the blackboard absorbs almost all colours of visible light and reflects very little light.

Verification / Alternative check:
Experiments in an optics laboratory show that black surfaces heat up more in sunlight because they absorb a wide range of wavelengths rather than reflecting them. This is why black objects often feel hotter in the Sun compared to white objects. This absorption behaviour matches the explanation that black materials take in most of the light energy and reflect little back to the observer. Thus the black appearance of a blackboard is consistent with strong absorption of visible light.

Why Other Options Are Wrong:
Option a suggests that the blackboard reflects black colour, but black is the absence of visible light and is not a colour that can be reflected in the usual sense.
Option b states that it absorbs only black colour, which is incorrect because black is not a specific wavelength that can be selectively absorbed; instead, many wavelengths are absorbed.
Option c claims that it reflects all colours, which would make the board appear white or very bright rather than black.

Common Pitfalls:
Students sometimes imagine black as a special colour that is emitted or reflected, similar to red or blue, which leads to confusion. Another common mistake is to think that white and black are themselves single colours instead of combinations of reflection and absorption. Remembering that white corresponds to reflection of most wavelengths and black corresponds to absorption of most wavelengths helps avoid these errors.

Final Answer:
A blackboard appears black because it absorbs almost all the colours of visible light and reflects very little light to the eye.