Wavelength1/0.02 = 50m.

Frequency: 50 Hz

Period: 1/50 = 0.02 seconds.

Wave speed: 50 * 50 = 2500m/s.

Amplitude: 0. 1

25. Bernoulli equation: p+1/2ρ v 2+ρ GH = c.

Because it is a horizontal tube, h=0.

The result is: p1+1/2ρ v12 = p2+1/2ρ v22.

p 1=p0= 10^5 pa,v 1= 1m/s,v2=2m/s,ρ= 10^3 kg/m^3

Pennsylvania P2 = 9.85 *10 4

Because the pressure is less than the standard pressure P0, if there is an opening here, water will not flow out.

26. The polarization angle of the first sheet is 0, and the linear polarization with the intensity of 1/2I0 is obtained after the natural light passes through.

The polarization angle of the second block is π/6, and the intensity after passing (COS π/6) 2 * 1/2I0 = 3/8I0.

The included angle between the third piece and the second piece is also π/6, and the passing strength is (COS π/6) 2 * 3/8I0 = 9/32I0.

The included angle between the fourth block and the third block is also π/6, and the passing strength (COS π/6) 2 * 9/32I0 = 27/128I0.

27.x=0.5cos(4πt-π/6) m

Amplitude: 0.5m.

Initial phase: -π/6

Period: 2π/4π = 0.5s

Frequency: 4π/2π=2 Hz

Angular frequency: 4π

28. In front of the first mirror, the direction is negative, and in the opposite direction, it is positive.

F1=10cm, l10cm =-20cm.

1/l 1 1- 1/l 10 = 1/f 1

Get: L 1 1=20 cm.

Magnification: β1= l1/l10 =-1.

F2 =-10cm, L20 = l11-d = 20-15 = 5cm.

1/L2 1- 1/L20 = 1/F2

L2 1= 10 cm is obtained.

Magnification: β2=L2 1/L20=2.

System magnification: β 1*β2=-2.

It's like a handstand, magnifying the real image twice.