Which of the following practical units of length is not correct?

Options

A. 1 fermi = `10^(-15)`m
B. 1 astronomical unit = `1.496xx10^(11)`m
C. 1 parsec = 3.26 light year
D. 1 light year = `9.46xx10^(12)`m

Answer

To determine which of the following practical units of length is not correct, we will evaluate each option provided.

1. **Fermi**:
- A fermi is defined as \(10^{-15}\) meters.
- This is a correct unit of length used primarily in nuclear physics.

2. **Astronomical Unit (AU)**:
- One astronomical unit is defined as the average distance from the Earth to the Sun, which is approximately \(1.496 \times 10^{11}\) meters.
- This is also a correct unit of length.

3. **Parsec**:
- A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond.
- It is equivalent to about \(3.26\) light years.
- In meters, one parsec is approximately \(3.086 \times 10^{16}\) meters, which is correct.

4. **Light Year**:
- A light year is the distance that light travels in one year, which is approximately \(9.46 \times 10^{12}\) meters.
- The statement that \(1 \text{ light year} = 9.46 \times 10^{15}\) meters is incorrect, as it should be \(9.46 \times 10^{12}\) meters.

After evaluating all the options, we can conclude that the incorrect unit of length is:

**Answer**: Option 4: \(9.46 \times 10^{15}\) meters (this is not correct).

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