A star emits light of `5500 Å` wavelength. Its appears blue to an observer on the earth, it means

Options

A. Star is going away from the earth
B. Star is stationary
C. Star is coming towards earth
D. None of the above

Answer

To solve the problem, we need to analyze the information given about the star's emitted light and its observed color. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Wavelength of the Emitted Light
The star emits light with a wavelength of \(5500 \, \text{Å}\) (angstroms).

### Step 2: Understand the Color Perception
The light emitted by the star appears blue to an observer on Earth. In the visible spectrum, blue light has a wavelength range approximately between \(4500 \, \text{Å}\) and \(4850 \, \text{Å}\).

### Step 3: Compare Emitted Wavelength with Blue Wavelength Range
The emitted wavelength of \(5500 \, \text{Å}\) is outside the blue range. Since the observer sees the star as blue, it indicates that the wavelength has decreased.

### Step 4: Apply the Doppler Effect Concept
According to the Doppler effect, when a light source moves towards an observer, the wavelength of the emitted light decreases (blue shift). Conversely, if the source moves away, the wavelength increases (red shift).

### Step 5: Conclude the Movement of the Star
Since the star emits light at \(5500 \, \text{Å}\) but appears blue (indicating a shorter wavelength), we conclude that the star must be moving towards the observer on Earth. Therefore, the correct option is that the star is coming towards the Earth.

### Final Answer
The star is coming towards the Earth (Option C).

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