Assertion (A) : When ordinary light is incident on thin oil film on the surface of water, different colours are seen.
Reason (R) : White light is composed of seven different colours.

Options

A. Both A and R true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

To analyze the given assertion and reason, we will break down each component step by step.

### Step 1: Understand the Assertion
The assertion states that when ordinary light is incident on a thin oil film on the surface of water, different colors are seen. This phenomenon occurs due to the interference of light waves.

**Hint for Step 1:** Consider how light behaves when it encounters different mediums, such as oil and water.

### Step 2: Understand the Reason
The reason states that white light is composed of seven different colors. This is a true statement, as white light can be dispersed into its constituent colors (red, orange, yellow, green, blue, indigo, violet) through a prism or similar medium.

**Hint for Step 2:** Recall the concept of white light and its spectrum.

### Step 3: Analyze the Relationship Between Assertion and Reason
While both the assertion and reason are true, the reason does not provide a correct explanation for the assertion. The different colors seen in the oil film are primarily due to the phenomenon of interference, where light waves reflect off the top and bottom surfaces of the oil film, causing constructive and destructive interference.

**Hint for Step 3:** Think about how interference differs from simple color composition.

### Step 4: Conclusion
Since both the assertion and reason are true, but the reason does not correctly explain the assertion, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.

**Final Answer:** Both assertion and reason are true, but the reason is not the correct explanation for the assertion.