# Physics 1161 PreLecture 17 Refraction of Light Lenses

• Slides: 21

Physics 1161: Pre-Lecture 17 Refraction of Light Lenses • sections 26 -3 – 26 -5

Light Doesn’t Just Bounce It Also Refracts! Reflects: Bounces qi = qr qi qr Refracts: Bends q 1 q 2 n 1 n 2 n 1 sin(q 1)= n 2 sin(q 2)

Index of Refraction 186, 000 mi/s (3 x 108 m/s): it’s not just a good idea, it’s the law! Speed of light in vacuum Speed of light in medium Index of refraction so always!

Indices of Refraction

Snell’s Law When light travels from one medium to another the speed changes v = c/n, but the frequency is constant. So the light bends: n 1 sin(q 1)= n 2 sin(q 2)

Shape of a Lens

Types of Lenses

Lens Terms

Three Rays to Locate Image • Ray parallel to axis bends through the focus. • Ray through the focus bends parallel to axis. • Ray through center of lens passes straight through.

Characterizing the Image • Images are characterized in the following way 1. Virtual or Real 2. Upright or Inverted 3. Reduced, Enlarged, Same Size

Object Beyond 2 f • Image is – Real – Inverted – Reduced

Object at 2 f • Image is – Real – Inverted – Same size

Object Between 2 f and f • Image is – Real – Inverted – Enlarged

Object at F • No Image is Formed!

Object Closer than F • Image is – Virtual – Upright – Enlarged

Converging Lens Images

Concave (Diverging) Lens • Ray parallel to axis refracts as if it comes from the first focus. • Ray which lines up with second focus refracts parallel to axis. • Ray through center of lens doesn’t bend.

Image Formed by Concave Lens • Image is always – Virtual – Upright – Reduced

Concave Lens Image Distance • As object distance decreases – Image size increases

Image Characteristics • CONVEX LENS – IMAGE DEPENDS ON OBJECT POSITION – Beyond F: Real; Inverted; Enlarged, Reduced, or Same Size – Closer than F: Virtual, Upright, Enlarged – At F: NO IMAGE • CONCAVE LENS – IMAGE ALWAYS SAME – Virtual – Upright – Reduced

Lens Equations do di • convex: f > 0; concave: f < 0 • do > 0 if object on left of lens • di > 0 if image on right of lens otherwise di < 0 • ho & hi are positive if above principal axis; negative below