Assertion and reason

Exercise and reference answers

Questions

1. Reason

   Total internal reflection needs travel from a denser medium to a rarer one.

   Verdict

   Both are true; the reason explains the assertion.

   Assertion

   Light cannot be totally internally reflected going from air into glass.

2. Reason

   Its coating has a lower refractive index, so light keeps reflecting internally.

   Verdict

   Both are true; the reason explains the assertion.

   Assertion

   An optical fibre carries light around bends.

3. Reason

   Focal length is roughly half the radius of curvature.

   Verdict

   The assertion is false and the reason is true.

   Assertion

   A concave mirror has a positive focal length.

4. Reason

   Power is the reciprocal of focal length, and those reciprocals add.

   Verdict

   Both are true; the reason explains the assertion.

   Assertion

   Powers of lenses in contact simply add.

Explanation