# Reflection in a Plane Mirror

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1. Jul 26, 2016

### Coco Hwang

1) If the angle of incidence of a ray of light to mirror is 50 degrees, what is the angle of reflection from the mirror?

2) If the angle of incidence of a ray of light to a mirror is 20 degrees, what angle does the light ray make with the mirror when it reflects?

3) If a ray of light makes an angle of 58 degrees with a mirror, what is the angle between the incident ray and the reflected ray?

The attempt - I'm having a hard time understanding the concept of mirrors, does the angle of reflection always equal to the angle of incidence?

Thank you!

2. Jul 26, 2016

### haruspex

Yes.

3. Jul 26, 2016

### Coco Hwang

thank you!

would this also apply for light rays?

4. Jul 26, 2016

### haruspex

Also? What were we discussing if not light rays?

5. Jul 26, 2016

### Coco Hwang

Sorry, I was not aware that angle of reflection and light rays were the same thing..

6. Jul 26, 2016

### haruspex

They're not the same thing. Light rays are reflected in mirrors. The angle of reflection is an attribute of that reflection.
Although "reflection" generally refers to light, we can use the term in other contexts, figuratively.
But in the context of the present question, we are definitely discussing reflection of light rays.

7. Jul 26, 2016

### Coco Hwang

Oh I see, thank you for your detailed response! So in this situation, would the answers for both questions #1 and #2 be the same as their angle of incidence?

8. Jul 26, 2016

### haruspex

For #1, yes, but for #2 you need to think carefully about how the following are defined:
• Angle of incidence
• Angle of reflection
• Angle between the light ray and the mirror

9. Jul 26, 2016

### Coco Hwang

ok so for #2, would I subtract 20 degrees from 90 degrees? ( 70 degrees ) Or do I just multiply 20 two times ( 40 degrees )

10. Jul 26, 2016

### haruspex

One of those is correct. Which one and why?

11. Jul 26, 2016

### Coco Hwang

Ummm, is it subtracting 20 from 90? Because the angle of incidence is 90 so I subtract the angle made from the reflection from it?

12. Jul 26, 2016

### haruspex

Yes, but why? How is the angle of reflection defined, geometrically? How is that different from the angle the reflection makes to the mirror?