total internal reflection: experiments for kids

Light Touch

Total Internal Reflection: Experiments for Kids I c a n r e m e m b e r w h e n r o t a r y p h o n e s w e r e the n o r m a n d w e h a d to f u s s w i t h b i g r a b b i t - e a r e d a n t e n n a s to u n f u z z o u r t e l e v i s i o n p i c t u r e . S o m e of u s a re o l d e n o u g h ( t h o u g h I 'm no t ) to e v e n r e m e m b e r w h a t l i f e w a s l i k e w h e n t e l e v i s i o n s w e r e a l u x u r y . T e c h ­n o l o g i c a l a d v a n c e s m a k e o l d t e c h n o l ­o g y o b s o l e t e . A n d i t ' s h a p p e n i n g fas te r a n d fas te r . A s k a n y 1 1 - y e a r - o l d w h a t t h e i r f a v o r i t e N i n t e n d o g a m e i s , a n d " P o n g " ( p o p u l a r i n t he m i d -1970s) w i l l p r o b a b l y n o t b e the a n ­s w e r . T h e n e x t g e n e r a t i o n o f t e c h n o l o g y i s b e i n g g i v e n a g i a n t b o o s t b y a s c i e n t i f i c p r i n c i p l e k n o w n as t o ta l i n t e r n a l r e f l e c t i o n (T IR ) . F i ­b e r o p t i c s — a t e c h n o l o g y i n w h i c h l i g h t t r a v e l s t h r o u g h g l a s s f i b e r s — i s o n e o f the m o s t i m p o r t a n t a p p l i c a ­t ions of T IR , a n d th is t echno logy is r e v o ­l u t i o n i z i n g o u r c o m m u n i c a t i o n s a n d c o m p u t e r s y s t e m s , as w e l l as a p ­p r o a c h e s to m e d i c i n e . In a f e w y e a r s , o u r c h i l d r e n w i l l b e w o n d e r i n g h o w i n the w o r l d w e e v e r go t a l o n g w i t h o u t th is n e w t e c h n o l o g y . H e r e are s o m e ex­c i t i n g e x p e r i m e n t s that h e l p e x p l a i n h o w T I R a n d f ibe r op t i c s w o r k .

S E N D L IGHT A R O U N D A

M E A S U R I N G C U P

T h i s e x p e r i m e n t g i v e s y o u a f i r s t - h a n d l o o k a t f i b e r o p t i c s . Y o u w i l l n e e d a g l a s s m e a s u r i n g c u p , a b o w l o f w a t e r , a n d a f l a s h l i g h t . S h i n e the f l a s h l i g h t d o w n i n t o the r i m o f the c u p . L o o k i n t o the e d g e o f the r i m tha t is o p p o ­s i te to the e d g e tha t the l i g h t i s e n t e r ­i n g , a n d y o u s h o u l d see the l i g h t f r o m the f l a s h l i g h t .

W h i l e y o u k e e p y o u r e y e s o n the e x i t i n g l i g h t , d i p the b o t t o m o f the c u p i n t o the w a t e r . M o s t o f the l i g h t i s n o w e x t i n g u i s h e d .

Y o u h a v e j us t t u r n e d y o u r m e a ­s u r i n g c u p i n t o a f i b e r o p t i c ! T h e l i g h t tha t en te rs the c u p t o t a l l y i n t e r n a l l y re f l ec ts o f f o f the i n s i d e e d g e s o f i t . It b o u n c e s t h r o u g h t h e c u p , t u r n s a r o u n d the c o r n e r s , a n d e x i t s b a c k at

the t o p o f the r i m . W h e n the c u p i s d i p p e d i n t o the w a t e r , T I R c a n n o t take p l a c e b e c a u s e the s p e e d o f l i g h t i n w a t e r is v e r y n e a r the s p e e d o f l i g h t i n t he g l a s s . A s a r e s u l t , t he l i g h t p a s s e s t h r o u g h the e d g e o f the g l a s s i n t o the w a t e r .

M A K E A COIN D I S A P P E A R

F o r t h i s e x p e r i m e n t y o u w i l l n e e d a p e n n y , a n o p a q u e c e r e a l b o w l f i l l e d w i t h w a t e r , a n d a s m a l l , c l e a r g l a s s . P l a c e the c o i n i n the b o t t o m o f the b o w l o f w a t e r , a n d o b s e r v e i t at a g r a z i n g a n g l e to the w a t e r . L o w e r the e m p t y g l a s s , u p s i d e d o w n , o v e r t he c o i n . K e e p the g l a s s s t r a i g h t so tha t n o w a t e r g e t s i n t o i t . S t a r e a t t he p e n n y t h r o u g h the s i d e o f the g l a s s as i t i s l o w e r e d , a n d the c o i n w i l l v a n i s h b e f o r e y o u r e y e s .

W h e n l i g h t g o e s f r o m o n e m e ­d i u m to a n o t h e r , i ts s p e e d c h a n g e s a b r u p t l y . If the l i g h t s t r i k i n g the i n ­t e r f ace o f the t w o s u b s t a n c e s i s at a n a n g l e the c h a n g e i n s p e e d w i l l c a u s e the l i g h t to b e n d o r r e f r ac t . T h e a n g l e b e t w e e n the i n c o m i n g l i g h t r a y a n d l i n e p e r p e n d i c u l a r t o t he i n t e r f a c e ( c a l l e d the n o r m a l ) i s c a l l e d the angle of incidence, a n d the s t e e p e r tha t a n g l e , the m o r e the l i g h t r e f r a c t s . A s l i g h t ' s s p e e d c h a n g e s b e t w e e n m e d i a f r o m s l o w to fas t (i.e., i t t r a v e l s f r o m w a t e r to a i r ) , i t b e n d s t o w a r d the b o u n d a r y s u r f a c e a n d a w a y f r o m the n o r m a l to tha t s u r f a c e . W h e n the i n c i d e n t l i g h t

h i t s t he b o u n d a r y at j us t the r i g h t a n g l e , the r e f r a c t e d l i g h t t r a v e l s a l o n g the b o u n d a r y s u r f a c e b e t w e e n the t w o m a t e r i a l s . If the l i g h t s t r i k e s the s u r ­f ace at a n y s t e e p e r a n g l e , a l l o f the l i g h t i s r e f l e c t e d at the b o u n d a r y . T h i s r e f l e c t e d l i g h t i s p r o d u c e d b y t o t a l i n t e r n a l r e f l e c t i o n , the p r i n c i p l e tha t g o v e r n s h o w f i b e r o p t i c s w o r k .

A s the g l a s s i s l o w e r e d a b o v e the

p e n n y , t he w a t e r l e v e l i s c h a n g e d . N o w , f o r the l i g h t f r o m the p e n n y to get o u t o f the b o w l , i t m u s t r e f r ac t at t w o w a t e r - a i r i n t e r f a c e s . W h e n the l i g h t r e f r ac t s at the f i r s t l e v e l o f w a ­te r , i t s a n g l e i s c h a n g e d s i g n i f i c a n t l y so tha t i t i s t o t a l l y r e f l e c t e d at the s e c o n d l e v e l o f w a t e r . T h e l i g h t n e v e r m a k e s i t o u t o f the b o w l , a n d y o u r e y e s n e v e r see t he c o i n .

O n e o f t he k e y e l e m e n t s i n the a d v a n c e m e n t o f t e c h n o l o g y i s c o m ­m u n i c a t i o n . J u s t t h i n k , 150 y e a r s a g o p e o p l e t a l k e d to e a c h o t h e r b y r e a l h o r s e p o w e r o v e r the p o n y e x p r e s s , a n d n o w w e a re s e n d i n g o u r v o i c e s at l i g h t s p e e d . F i b e r o p t i c c a b l e s c a n s e n d d a t a f r o m o n e p l a c e to a n o t h e r m u c h m o r e e f f i c i e n t l y t h a n the c o n ­v e n t i o n a l c o p p e r w i r e , a n d t h i s t e c h ­n o l o g y w i l l h e l p u s l e a p f r o g i n t o the 21st c e n t u r y .

JOSH COBB IS an optical systems engi­neer/lens designer with IBM's Optical Development Services, Poughkeepsie, N.Y., and co-author of L i g h t A c t i o n , a children's book on optics. Send light around a measuring cup

Light takes new path and is totally internally reflected. Coin is no longer visible.

44 OPTICS & PHOTONICS N E W S / A U G U S T 1994