FDTD pulse hitting dielectric medium

import numpy as np
from math import exp
from matplotlib import pyplot as plt

ke = 200
ex = np.zeros(ke)
hy = np.zeros(ke)

t0 = 40
spread = 12

boundary_low = [0, 0]
boundary_high = [0, 0]

# create dielectric profile
cb = np.ones(ke)
cb = 0.5 * cb
cb_start = 100
epsilon = 4
cb[cb_start:] = 0.5 / epsilon

nsteps = 540

# dictionary to keep track of desired points for plotting
plotting_points = [
    {'num_steps': 100, 'data_to_plot': None, 'label': ''},
    {'num_steps': 220, 'data_to_plot': None, 'label': ''},
    {'num_steps': 320, 'data_to_plot': None, 'label': ''},
    {'num_steps': 540, 'data_to_plot': None, 'label': 'FDTDcells'}
]

# main FDTD Loop
for time_step in range(1, nsteps + 1):

    # calculate Ex field
    for k in range(1, ke):
        ex[k] = ex[k] + cb[k] * (hy[k - 1] - hy[k])

    # put a gaussian pulse at the low end
    pulse = exp(-0.5 * ((t0 - time_step) / spread) ** 2)
    ex[5] = pulse + ex[5]

    ex[0] = boundary_low.pop(0)
    boundary_low.append(ex[1])

    ex[ke - 1] = boundary_high.pop(0)
    boundary_high.append(ex[ke - 2])

    # calculate the Hy field
    for k in range(ke - 1):
        hy[k] = hy[k] + 0.5 * (ex[k] - ex[k + 1])

    # solve data at certain points for later plotting
    for plotting_point in plotting_points:
        if time_step == plotting_point['num_steps']:
            plotting_point['data_to_plot'] = np.copy(ex)

# plot the outputs
plt.rcParams['font.size'] = 12
fig = plt.figure(figsize=(8, 7))

def plot_e_field(data, timestep, epsilon, cb, label):
    plt.plot(data, color='k', linewidth=1)
    plt.ylabel('E$_x$', fontsize=14)
    plt.xticks(np.arange(0, 199, step=20))
    plt.xlim(0, 199)
    plt.yticks(np.arange(-0.5, 1.2, step=0.5))
    plt.ylim(-0.5, 1.2)
    plt.text(70, 0.5, 'T = {}'.format(timestep), horizontalalignment='center')
    plt.plot((0.5 / cb - 1) / 3, 'k--', linewidth=0.75)
    # math on cb above just for scaling
    plt.text(170, 0.5, 'Eps = {}'.format(epsilon), horizontalalignment='center')
    plt.xlabel('{}'.format(label))

# plot E field at each time step saved earlier
for subplot_num, plotting_point in enumerate(plotting_points):
    ax = fig.add_subplot(4, 1, subplot_num + 1)
    plot_e_field(plotting_point['data_to_plot'],
                 plotting_point['num_steps'], epsilon, cb,
                 plotting_point['label'])

plt.subplots_adjust(bottom=0.1, hspace=0.45)

plt.show()